Constraints on locations of reference blocks for intra block copy prediction

ABSTRACT

When encoding/decoding a current block of a current picture using intra block copy (“BC”) prediction, the location of a reference block is constrained so that it can be entirely within an inner search area of the current picture or entirely within an outer search area of the current picture, but cannot overlap both the inner search area and the outer search area. In some hardware-based implementations, on-chip memory buffers sample values of the inner search area, and off-chip memory buffers sample values of the outer search area. By enforcing this constraint on the location of the reference block, an encoder/decoder can avoid memory access operations that are split between on-chip memory and off-chip memory when retrieving the sample values of the reference block. At the same time, a reference block close to the current block may be used for intra BC prediction, helping compression efficiency.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional PatentApplication No. 62/612,293, filed Dec. 29, 2017, the disclosure of whichis hereby incorporated by reference.

BACKGROUND

Engineers use compression (also called source coding or source encoding)to reduce the bit rate of digital video. Compression decreases the costof storing and transmitting video information by converting theinformation into a lower bit rate form. Decompression (also calleddecoding) reconstructs a version of the original information from thecompressed form. A “codec” is an encoder/decoder system.

Over the last 30 years, various video codec standards have been adopted,including the ITU-T H.261, H.262 (MPEG-2 or ISO/IEC 13818-2), H.263,H.264 (MPEG-4 AVC or ISO/IEC 14496-10), and H.265 (HEVC or ISO/IEC23008-2) standards, the MPEG-1 (ISO/IEC 11172-2) and MPEG-4 Visual(ISO/IEC 14496-2) standards, and the SMPTE 421M (VC-1) standard. Morerecently, various extensions to the H.265 standard (e.g., for scalablevideo coding/decoding, for coding/decoding of video with higher fidelityin terms of sample bit depth or chroma sampling rate, for screencontent) have also been adopted. A video codec standard typicallydefines options for the syntax of an encoded video bitstream, detailingparameters in the bitstream when particular features are used inencoding and decoding. In many cases, a video codec standard alsoprovides details about the decoding operations a decoder should performto achieve conforming results in decoding. Aside from codec standards,various codec formats such as VP8, VP9, and AV1 (currently underdevelopment) define other options for the syntax of an encoded videobitstream and corresponding decoding operations.

Intra block copy (“BC”) is a prediction mode in the H.265 standard. Anintra BC prediction mode is also under consideration for the AV1 format.For intra BC prediction, the sample values of a current block of apicture are predicted using previously reconstructed sample values of areference block in the same picture. A block vector (“BY”) valueindicates a displacement from the current block to the reference blockin the same picture. The BV value can be signaled in the bitstream. Inparticular, intra BC prediction can be effective for encoding screencontent.

In intra BC prediction mode for the H.265 standard, a reference block isallowed to be anywhere in previously reconstructed content in the samepicture, even right next to the current block. This can make intra BCprediction mode very effective in terms of lowering bit rate andimproving quality, since a nearby reference block may closely match thecurrent block. In hardware implementations of intra BC prediction modefor the H.265 standard, however, memory access operations for areference block can be slow and inefficient. A hardware-based encoder orhardware-based decoder typically stores sample values in fast “on-chip”memory (such as a fast cache) during reconstruction of those samplevalues, when the sample values are accessed frequently. Because size ofthe on-chip memory is limited, the hardware-based encoder/decodereventually transfers a batch of reconstructed sample values to slower“off-chip” memory (such as dynamic random access memory). In theoff-chip memory, the reconstructed sample values may still be accessedfor intra BC prediction mode or other prediction, but memory accessoperations are slower. In particular, in hardware implementations ofintra BC prediction mode for the H.265 standard, memory accessoperations can be slow and inefficient when one part of a referenceblock (close to the current block) is stored in on-chip memory but therest of the reference block (further from the current block) isseparately stored in off-chip memory.

In contrast, in some current implementations of intra BC prediction modefor the AV1 format, a reference block is constrained to be outside a“superblock” that includes the current unit being encoded/decoded withintra BC prediction, or the reference block is constrained to be outsidethe superblock that includes the current unit and also outside theprevious superblock. With such a constraint on locations of referenceblocks, intra BC prediction is more “hardware friendly”—slow,inefficient memory access operations can be avoided in hardwareimplementations—but intra BC prediction may be less effective in termsof lowering bit rate and improving quality.

SUMMARY

In summary, the detailed description presents innovations in intra blockcopy (“BC”) prediction. For example, when encoding/decoding a currentblock of a current picture using intra BC prediction, the location of areference block for the intra BC prediction is constrained so that thereference block can be entirely within an inner search area of thecurrent picture or entirely within an outer search area of the currentpicture, but the reference block cannot overlap both the inner searcharea and the outer search area. In some example implementations, on-chipmemory buffers sample values of the inner search area, and off-chipmemory buffers sample values of the outer search area. By enforcing thisconstraint on the location of the reference block, an encoder/decodercan avoid memory access operations that are split between on-chip memoryand off-chip memory when retrieving the sample values of the referenceblock. At the same time, a reference block close to the current block(within the inner search area) may be used for intra BC prediction,which can improve the effectiveness of intra BC prediction in terms oflowering bit rate and improving quality.

According to one aspect of the innovations described herein, a videoencoder or image encoder encodes a current picture to produce encodeddata and outputs the encoded data as part of a bitstream. During theencoding, for a current unit of the current picture, the encoderdetermines a block vector (“BY”) value associated with the current unitand performs intra BC prediction for a given block of the current unitusing the BV value associated with the current unit. The BV valueindicates a displacement from the given block of the current unit to areference block in the current picture. When it determines the BV value,the encoder enforces a constraint that (a) allows the reference block tobe entirely within an inner search area of the current picture and (b)allows the reference block to be entirely within an outer search area ofthe current picture but (c) prohibits the reference block fromoverlapping both the inner search area and the outer search area. Thatis, the reference block can be located entirely within the outer searcharea, or the reference block can be located entirely within the innersearch area. The reference block cannot, however, be partially locatedin the inner search area and partially located in the outer search area.For example, the inner search area is a root superblock that includesthe current unit, and the outer search area is other previouslyencoded/reconstructed parts of the current picture. In some exampleimplementations, a first buffer in on-chip memory stores sample valuesof the inner search area, and a second buffer in off-chip memory storessample values of the outer search area.

A boundary line or boundary region can separate the inner search areafrom the outer search area. To enforce the constraint, the encoder cancheck that the reference block does not cross the boundary line orboundary region. Or, to enforce the constraint, the encoder can checkthat the reference block is completely contained within the inner searcharea or completely contained within the outer search area.

According to another aspect of the innovations described herein, a videodecoder or image decoder receives encoded data as part of a bitstreamand decodes a current picture using the encoded data. The encoded datasatisfies bitstream conformance requirements. During the decoding, for acurrent unit of the current picture, the decoder reconstructs a BV valueassociated with the current unit and performs intra BC prediction for agiven block of the current unit using the BV value associated with thecurrent unit. The BV value indicates a displacement from the given blockof the current unit to a reference block in the current picture.According to one of the bitstream conformance requirements, the BV valueis constrained so that (a) the reference block is allowed to be entirelywithin an inner search area of the current picture and (b) the referenceblock is allowed to be entirely within an outer search area of thecurrent picture but (c) the reference block is prohibited fromoverlapping both the inner search area and the outer search area. Thatis, the reference block can be located entirely within the outer searcharea, or the reference block can be located entirely within the innersearch area. The reference block cannot, however, be partially locatedin the inner search area and partially located in the outer search area.

The innovations can be implemented as part of a method, as part of acomputing system configured to perform the method or as part of atangible computer-readable media storing computer-executableinstructions for causing a computing system to perform the method. Thevarious innovations can be used in combination or separately. Thissummary is provided to introduce a selection of concepts in a simplifiedform that are further described below in the detailed description. Thissummary is not intended to identify key features or essential featuresof the claimed subject matter, nor is it intended to be used to limitthe scope of the claimed subject matter. The foregoing and otherobjects, features, and advantages of the invention will become moreapparent from the following detailed description, which proceeds withreference to the accompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of an example computer system in which somedescribed embodiments can be implemented.

FIGS. 2a and 2b are diagrams of example network environments in whichsome described embodiments can be implemented.

FIG. 3 is a diagram of an example encoder system, and FIGS. 4a and 4bare diagrams illustrating an example video encoder, in conjunction withwhich some described embodiments can be implemented.

FIG. 5 is a diagram of an example decoder system, and FIG. 6 is adiagram illustrating an example video decoder, in conjunction with whichsome described embodiments can be implemented.

FIGS. 7 and 8 are diagrams illustrating aspects of intra BC predictionfor a current block of a picture.

FIGS. 9a, 9b , and 10-12 are diagrams illustrating aspects of exampleconstraints on locations of reference blocks for intra BC prediction.

FIG. 13 is a flowchart illustrating a generalized technique for encodingwith constraints on locations of reference blocks for intra BCprediction mode.

FIG. 14 is a flowchart illustrating an example technique for encodingunits of superblocks of a current picture with intra BC prediction.

FIGS. 15a and 15b are flowcharts illustrating example techniques fordetermining a BV value associated with a current unit, subject toconstraints on location of a reference block.

FIG. 16 is a flowchart illustrating a generalized technique for decodingwith constraints on locations of reference blocks for intra BCprediction mode.

DETAILED DESCRIPTION

The detailed description presents innovations in intra block copy (“BC”)prediction. In particular, when encoding/decoding a current block of acurrent picture using intra BC prediction, the location of a referenceblock for the intra BC prediction is constrained. For example, thereference block is constrained to be entirely within an inner searcharea of the current picture or entirely within an outer search area ofthe current picture, but the reference block cannot overlap both theinner search area and the outer search area. By enforcing thisconstraint on the location of the reference block, a hardwareencoder/decoder can avoid memory access operations that are splitbetween on-chip memory (buffering sample values of the inner searcharea) and off-chip memory (buffering sample values of the outer searcharea), while still permitting the reference block to be close to thecurrent block (within the inner search area).

Although operations described herein are in places described as beingperformed by a video encoder or video decoder, in many cases theoperations can be performed by another type of media processing tool(e.g., image encoder or image decoder).

Some of the innovations described herein are illustrated with referenceto terms specific to extensions of the AV1 format. The innovationsdescribed herein can also be implemented for other codec formats. Forexample, some of the innovations described herein can be implemented inextensions of the H.265 standard.

Many of the innovations described herein are adapted for encoding ordecoding of certain “artificially-created” video content such as screencontent. In general, screen content (also called screen capture video)is video that contains rendered text, computer graphics,animation-generated content or other similar types of content capturedwhen rendered to a computer display, as opposed to camera-captured videocontent only. Screen content typically includes repeated structures(e.g., graphics, text characters). Also, screen content typicallyincludes a small number of different colors, compared to “natural” videocaptured with a camera. Drastic changes from pixel-to-pixel and sharpedges are also common in screen content, compared to natural video.Screen content is often encoded in a format (e.g., YUV 4:4:4 or RGB4:4:4) with high chroma sampling resolution, although it may instead beencoded in a format with lower chroma sampling resolution (e.g., YUV4:2:0). Common scenarios for encoding/decoding of screen content includeremote desktop conferencing and encoding/decoding of graphical overlayson natural video or other “mixed content” video. Several of theinnovations described herein are adapted for encoding/decoding of screencontent or other artificially-created video. These innovations can alsobe used for natural video, but may not be as effective.

Many of the examples described herein involve intra BC prediction forblocks of luma sample values (luma blocks) and corresponding blocks ofchroma sample values (chroma blocks) for a picture in a YUV 4:4:4format, YUV 4:2:2, format or YUV 4:2:0 format. Chroma sample values maybe sub-sampled to a lower chroma sampling rate (e.g., for YUV 4:2:0format or YUV 4:2:2 format), or the chroma sample values may have thesame resolution as the luma sample values (e.g., for YUV 4:4:4 format).Alternatively, video can be organized according to another format (e.g.,RGB 4:4:4 format, GBR 4:4:4 format or BGR 4:4:4 format). In general, aprimary component block is a block of sample values for a primarycomponent (such as Y for a YUV format, or R for an RGB format), and asecondary component block is a block of sample values for a secondarycomponent (such as U or V for a YUV format, or G or B for an RGBformat).

More generally, various alternatives to the examples described hereinare possible. For example, some of the methods described herein can bealtered by changing the ordering of the method acts described, bysplitting, repeating, or omitting certain method acts, etc. The variousaspects of the disclosed technology can be used in combination orseparately. Different embodiments use one or more of the describedinnovations. Some of the innovations described herein address one ormore of the problems noted in the background. Typically, a giventechnique/tool does not solve all such problems.

I. Example Computer Systems.

FIG. 1 illustrates a generalized example of a suitable computer system(100) in which several of the described innovations may be implemented.The innovations described herein relate to intra BC prediction, which ingeneral involves prediction of the sample values of a current block of apicture using previously reconstructed sample values of a referenceblock in the same picture. Aside from its use in intra BC prediction,the computer system (100) is not intended to suggest any limitation asto scope of use or functionality, as the innovations may be implementedin diverse computer systems, including special-purpose computer systemsadapted for video/image encoding and/or decoding.

With reference to FIG. 1, the computer system (100) includes one or moreprocessing cores (110 . . . 11 x) and local memory (118) ofsystem-on-a-chip (“SoC”), application-specific integrated circuit(“ASIC”) or other integrated circuit. The processing core(s) (110 . . .11 x) are, for example, processing cores on a single chip, and executecomputer-executable instructions. The number of processing core(s) (110. . . 11 x) depends on implementation and can be, for example, 4 or 8.The local memory (118) may be volatile memory (e.g., registers, cache,RAM), non-volatile memory (e.g., ROM, EEPROM, flash memory, etc.), orsome combination of the two, accessible by the respective processingcore(s) (110 . . . 11 x).

The local memory (118) can store software (180) implementing one or moreinnovations for encoding/decoding with constraints on locations ofreference blocks for intra BC prediction, for operations performed bythe respective processing core(s) (110 . . . 11 x), in the form ofcomputer-executable instructions. In FIG. 1, the local memory (118) ison-chip memory such as one or more caches, for which access operations,transfer operations, etc. with the processing core(s) (110 . . . 11 x)are fast. The local memory (118) can include at least one buffer thatstores reconstructed sample values of an inner search area of a picture.The location of a reference block can be constrained so that thereference block is permitted to be entirely within the inner search areabut is not permitted to be only partially overlapping the inner searcharea.

The computer system (100) also includes processing cores (130 . . . 13x) and local memory (138) of a graphics processing unit (“GPU”). Thenumber of processing cores (130 . . . 13 x) of the GPU depends onimplementation. The processing cores (130 . . . 13 x) are, for example,part of single-instruction, multiple data (“SIMD”) units of the GPU. TheSIMD width n, which depends on implementation, indicates the number ofelements (sometimes called lanes) of a SIMD unit. For example, thenumber of elements (lanes) of a SIMD unit can be 16, 32, 64, or 128 foran extra-wide SIMD architecture. The local memory (138) may be volatilememory (e.g., registers, cache, RAM), non-volatile memory (e.g., ROM,EEPROM, flash memory, etc.), or some combination of the two, accessibleby the respective processing cores (130 . . . 13 x).

The local memory (138) can store software (180) implementing one or moreinnovations for encoding/decoding with constraints on locations ofreference blocks for intra BC prediction, for operations performed bythe respective processing cores (130 . . . 13 x), in the form ofcomputer-executable instructions such as shader code. In FIG. 1, thelocal memory (138) is on-chip memory such as one or more caches, forwhich access operations, transfer operations, etc. with the processingcores (130 . . . 13 x) are fast. The local memory (138) can include atleast one buffer that stores reconstructed sample values of an innersearch area of a picture. The location of a reference block can beconstrained so that the reference block is permitted to be entirelywithin the inner search area but is not permitted to be only partiallyoverlapping the inner search area.

The computer system (100) includes shared memory (120), which may bevolatile memory (e.g., RAM), non-volatile memory (e.g., ROM, EEPROM,flash memory, etc.), or some combination of the two, accessible by theprocessing core(s) (110 . . . 11 x, 130 . . . 13 x). The memory (120)stores software (180) implementing one or more innovations forencoding/decoding with constraints on locations of reference blocks forintra BC prediction, in the form of computer-executable instructions. InFIG. 1, the shared memory (120) is off-chip memory, for which accessoperations, transfer operations, etc. with the processing cores (110 . .. 11 x, 130 . . . 13 x) are slower. The shared memory (120) can includeat least one buffer that stores reconstructed sample values of an outersearch area of a picture. The location of a reference block can beconstrained so that the reference block is permitted to be entirelywithin the outer search area but is not permitted to be only partiallyoverlapping the outer search area.

Alternatively, the computer system (100) includes one or more processingcores of a central processing unit (“CPU”) and associated memory. Theprocessing core(s) of the CPU can execute computer-executableinstructions for one or more innovations for encoding/decoding withconstraints on locations of reference blocks for intra BC prediction.

The computer system (100) includes one or more network interface devices(140). The network interface device(s) (140) enable communication over anetwork to another computing entity (e.g., server, other computersystem). The network interface device(s) (140) can support wiredconnections and/or wireless connections, for a wide-area network,local-area network, personal-area network or other network. For example,the network interface device(s) can include one or more Wi-Fitransceivers, an Ethernet port, a cellular transceiver and/or anothertype of network interface device, along with associated drivers,software, etc. The network interface device(s) (140) convey informationsuch as computer-executable instructions, audio or video input oroutput, or other data in a modulated data signal over networkconnection(s). A modulated data signal is a signal that has one or moreof its characteristics set or changed in such a manner as to encodeinformation in the signal. By way of example, and not limitation, thenetwork connections can use an electrical, optical, RF, or othercarrier.

A camera input (150) accepts video input in analog or digital form froma video camera, which captures natural video. An audio input acceptsaudio input in analog or digital form from a microphone (150), whichcaptures audio.

The computer system (100) optionally includes a motion sensor/trackerinput (152) for a motion sensor/tracker, which can track the movementsof a user and objects around the user. For example, the motionsensor/tracker allows a user (e.g., player of a game) to interact withthe computer system (100) through a natural user interface usinggestures and spoken commands. The motion sensor/tracker can incorporategesture recognition, facial recognition and/or voice recognition.

A game controller input (154) accepts control signals from one or moregame controllers, over a wired connection or wireless connection. Thecontrol signals can indicate user inputs from one or more directionalpads, buttons, triggers and/or one or more joysticks of a gamecontroller. The control signals can also indicate user inputs from atouchpad or touchscreen, gyroscope, accelerometer, angular rate sensor,magnetometer and/or other control or meter of a game controller.

The computer system (100) optionally includes a media player (156) andvideo input (158). The media player (156) can play DVDs, Blu-ray disks,other disk media and/or other formats of media. The video input (158)can accept input video in analog or digital form (e.g., from a cableinput, HDMI input or other input).

A video output (160) provides video output to a display device. Thevideo output (160) can be an HDMI output or other type of output. Anaudio output (160) provides audio output to one or more speakers.

The storage (170) may be removable or non-removable, and includesmagnetic media (such as magnetic disks, magnetic tapes or cassettes),optical disk media and/or any other media which can be used to storeinformation and which can be accessed within the computer system (100).The storage (170) stores instructions for the software (180)implementing one or more innovations for encoding/decoding withconstraints on locations of reference blocks for intra BC prediction.

The computer system (100) may have additional features. For example, thecomputer system (100) includes one or more other input devices and/orone or more other output devices. The other input device(s) may be atouch input device such as a keyboard, mouse, pen, or trackball, ascanning device, or another device that provides input to the computersystem (100). The other output device(s) may be a printer, CD-writer, oranother device that provides output from the computer system (100).

An interconnection mechanism (not shown) such as a bus, controller, ornetwork interconnects the components of the computer system (100).Typically, operating system software (not shown) provides an operatingenvironment for other software executing in the computer system (100),and coordinates activities of the components of the computer system(100).

The innovations can be described in the general context ofcomputer-readable media. Computer-readable media are any availabletangible media that can be accessed within a computing environment. Byway of example, and not limitation, with the computer system (100),computer-readable media include memory (118, 120, 138), storage (170),and combinations thereof. The term computer-readable media does notencompass transitory propagating signals or carrier waves.

The innovations can be described in the general context ofcomputer-executable instructions being executed in a computer system ona target real or virtual processor. The computer-executable instructionscan include instructions executable on processing cores of ageneral-purpose processor to provide functionality described herein,instructions (e.g., for a host, driver, or wrapper layer) executable tocontrol a GPU or special-purpose hardware (e.g., SoC, ASIC) to providefunctionality described herein, instructions executable on processingcores of a GPU to provide functionality described herein, and/orinstructions executable on processing cores of a special-purposeprocessor (e.g., SoC, ASIC) to provide functionality described herein.In some implementations, computer-executable instructions can beorganized in program modules. Generally, program modules includeroutines, programs, libraries, objects, classes, components, datastructures, etc. that perform particular tasks or implement particularabstract data types. The functionality of the program modules may becombined or split between program modules as desired in variousembodiments. Computer-executable instructions for program modules may beexecuted within a local or distributed computing system.

The terms “system” and “device” are used interchangeably herein. Unlessthe context clearly indicates otherwise, neither term implies anylimitation on a type of computer system or device. In general, acomputer system or device can be local or distributed, and can includeany combination of special-purpose hardware and/or hardware withsoftware implementing the functionality described herein.

For the sake of presentation, the detailed description uses terms like“determine,” “receive” and “provide” to describe computer operations ina computing system. These terms denote operations performed by acomputer and should not be confused with acts performed by a humanbeing. The actual computer operations corresponding to these terms varydepending on implementation.

II. Example Network Environments.

FIGS. 2a and 2b show example network environments (201, 202) thatinclude video encoders (220) and video decoders (270). The encoders(220) and decoders (270) are connected over a network (250) using anappropriate communication protocol. The network (250) can include theInternet or another computer network.

In the network environment (201) shown in FIG. 2a , each real-timecommunication (“RTC”) tool (210) includes both an encoder (220) and adecoder (270) for bidirectional communication. A given encoder (220) canproduce output compliant with an extension of the AV1 format, VP8format, VP9 format, H.265 format, or another codec format, with acorresponding decoder (270) accepting encoded data from the encoder(220). The bidirectional communication can be part of a videoconference, video telephone call, or other two-party or multi-partycommunication scenario. Although the network environment (201) in FIG.2a includes two real-time communication tools (210), the networkenvironment (201) can instead include three or more real-timecommunication tools (210) that participate in multi-party communication.

A real-time communication tool (210) manages encoding by an encoder(220). FIG. 3 shows an example encoder system (300) that can be includedin the real-time communication tool (210). Alternatively, the real-timecommunication tool (210) uses another encoder system. A real-timecommunication tool (210) also manages decoding by a decoder (270). FIG.5 shows an example decoder system (500), which can be included in thereal-time communication tool (210). Alternatively, the real-timecommunication tool (210) uses another decoder system.

In the network environment (202) shown in FIG. 2b , an encoding tool(212) includes an encoder (220) that encodes video for delivery tomultiple playback tools (214), which include decoders (270). Theunidirectional communication can be provided for a video surveillancesystem, web camera monitoring system, remote desktop conferencingpresentation or other scenario in which video is encoded and sent fromone location to one or more other locations. Although the networkenvironment (202) in FIG. 2b includes two playback tools (214), thenetwork environment (202) can include more or fewer playback tools(214). In general, a playback tool (214) communicates with the encodingtool (212) to determine a stream of video for the playback tool (214) toreceive. The playback tool (214) receives the stream, buffers thereceived encoded data for an appropriate period, and begins decoding andplayback.

FIG. 3 shows an example encoder system (300) that can be included in theencoding tool (212). Alternatively, the encoding tool (212) uses anotherencoder system. The encoding tool (212) can also include server-sidecontroller logic for managing connections with one or more playbacktools (214). FIG. 5 shows an example decoder system (500), which can beincluded in the playback tool (214). Alternatively, the playback tool(214) uses another decoder system. A playback tool (214) can alsoinclude client-side controller logic for managing connections with theencoding tool (212).

III. Example Encoder Systems and Video Encoders.

FIG. 3 is a block diagram of an example encoder system (300) inconjunction with which some described embodiments may be implemented.The encoder system (300) can be a general-purpose encoding tool capableof operating in any of multiple encoding modes such as a low-latencyencoding mode for real-time communication, a transcoding mode, and ahigher-latency encoding mode for producing media for playback from afile or stream, or it can be a special-purpose encoding tool adapted forone such encoding mode. The encoder system (300) can be adapted forencoding of a particular type of content (e.g., screen content). Theencoder system (300) can be implemented as part of an operating systemmodule, as part of an application library, as part of a standaloneapplication, using GPU hardware, or using special-purpose hardware.Overall, the encoder system (300) receives a sequence of source videopictures (311) from a video source (310) and produces encoded data asoutput to a channel (390). The encoded data output to the channel (390)can include content encoded using intra BC prediction with constraintson locations of reference blocks as described herein.

The video source (310) can be a camera, tuner card, storage media,screen capture module, or other digital video source. The video source(310) produces a sequence of video pictures at a frame rate of, forexample, 30 frames per second. As used herein, the term “picture”generally refers to source, coded or reconstructed image data. Forprogressive-scan video, a picture is a progressive-scan video frame. Forinterlaced video, in example embodiments, an interlaced video framemight be de-interlaced prior to encoding. Alternatively, twocomplementary interlaced video fields are encoded together as a singlevideo frame or encoded as two separately-encoded fields. Aside fromindicating a progressive-scan video frame or interlaced-scan videoframe, the term “picture” can indicate a single non-paired video field,a complementary pair of video fields, a video object plane thatrepresents a video object at a given time, or a region of interest in alarger image. The video object plane or region can be part of a largerimage that includes multiple objects or regions of a scene.

An arriving source picture (311) is stored in a source picture temporarymemory storage area (320) that includes multiple picture buffer storageareas (321, 322, . . . , 32 n). A picture buffer (321, 322, etc.) holdsone source picture in the source picture storage area (320). After oneor more of the source pictures (311) have been stored in picture buffers(321, 322, etc.), a picture selector (330) selects an individual sourcepicture from the source picture storage area (320). The order in whichpictures are selected by the picture selector (330) for input to theencoder (340) may differ from the order in which the pictures areproduced by the video source (310), e.g., the encoding of some picturesmay be delayed in order, so as to allow some later pictures to beencoded first and to thus facilitate temporally backward prediction.Before the encoder (340), the encoder system (300) can include apre-processor (not shown) that performs pre-processing of the selectedpicture (331) before encoding. The pre-processing can include colorspace conversion into primary (e.g., luma) and secondary (e.g., chromadifferences toward red and toward blue) color components and resamplingprocessing (e.g., to reduce the spatial resolution of chroma components)for encoding.

Before encoding, video may be converted to a color space such as YUV. AYUV format includes a luma (or Y) component with sample valuesrepresenting approximate brightness values as well as multiple chroma(or U and V) components with sample values representing color-differencevalues. The precise definitions of the color-difference values (andconversion operations to/from YUV color space to another color spacesuch as RGB) depend on implementation. In general, as used herein, theterm YUV indicates any color space with a luma (or luminance) componentand one or more chroma (or chrominance) components, including Y′UV, YIQ,Y′IQ and YDbDr as well as variations such as YCbCr and YCoCg.

Chroma sample values may be sub-sampled to a lower chroma sampling rate,or chroma sample values may have the same resolution as luma samplevalues. In a YUV 4:4:4 format, chroma information is represented at thesame spatial resolution as luma information. Many commercially availablevideo encoders and decoders support a YUV 4:2:0 chroma sampling formator YUV 4:2:2 chroma sampling format. A YUV 4:2:0 format is a format thatsub-samples chroma information compared to a YUV 4:4:4 format, so thatchroma resolution is half that of luma resolution both horizontally andvertically. As a design principle, the decision to use a YUV 4:2:0format for encoding/decoding is premised on the understanding that, fortypical use cases such as encoding/decoding of natural, camera-capturedvideo content, viewers do not ordinarily notice significant visualdifferences between video encoded/decoded in a YUV 4:2:0 format andvideo encoded/decoded in a YUV 4:4:4 format. The compression advantagesfor the YUV 4:2:0 format, which has fewer samples per picture, aretherefore compelling. A YUV 4:2:2 format is a format that sub-sampleschroma information compared to a YUV 4:4:4 format, so that chromaresolution is half that of luma resolution horizontally.

Alternatively, video can be organized according to another format (e.g.,RGB 4:4:4 format, GBR 4:4:4 format or BGR 4:4:4 format) for encoding. Ingeneral, a primary component block is a block of sample values for aprimary component (such as Y for a YUV format, or R for an RGB format),and a secondary component block is a block of sample values for asecondary component (such as U or V for a YUV format, or G or B for anRGB format).

The encoder (340) encodes the selected picture (331) to produce a codedpicture (341). FIGS. 4a and 4b are a block diagram of a generalizedvideo encoder (400) in conjunction with which some described embodimentsmay be implemented. The encoder (400) receives the selected, currentpicture (331) from a sequence of video pictures as an input video signal(405) and produces encoded data for the coded picture (341) in a codedvideo bitstream (495) as output. The codec format of the coded videobitstream (495) can be an extension of AV1 format, VPx format (e.g.,VP8, VP9), H.26x format (e.g., H.261, H.262, H.263, H.264, H.265),Windows Media Video format, VC-1 format, MPEG-x format (e.g., MPEG-1,MPEG-2, or MPEG-4), or another codec format.

The encoder (400) compresses pictures using intra-picture coding andinter-picture coding. Many of the components of the encoder (400) areused for both intra-picture coding and inter-picture coding. The exactoperations performed by those components can vary depending on the codecformat and the type of information being compressed.

A tiling module (410) optionally partitions a picture into multipletiles of the same size or different sizes. For example, the tilingmodule (410) splits the picture along tile rows and tile columns that,with picture boundaries, define horizontal and vertical boundaries oftiles within the picture, where each tile is a rectangular region. Tilesare often used to provide options for parallel processing. Inimplementations that extend the AV1 format, the encoder (400) can alsopartition a picture into segments, and parameters of blocks (orsuperblocks) of a given segment can be collectively signaled for thegiven segment, which can improve compression efficiency. Inimplementations that extend the H.265 format, the encoder (400)partitions a picture into one or more slices, where each slice includesone or more slice segments. Thus, a slice can be an entire picture or asection of the picture. A slice can be decoded independently of otherslices in a picture, which improves error resilience.

The content of a picture (or tile, slice, etc.) is further partitionedinto blocks of sample values for purposes of encoding and decoding. Theencoder (400) is block-based and uses a block format that depends onimplementation. Blocks may be further sub-divided at different stages,e.g., at the prediction, frequency transform and/or entropy encodingstages. For example, a picture can be divided into 256×256 blocks,128×128 blocks, 64×64 blocks, 32×32 blocks, or 16×16 blocks, which canin turn be divided into smaller blocks of sample values at differentstages of coding and decoding.

In implementations of encoding for extensions of the AV1 format, forexample, the encoder (400) partitions a picture (or tile) intosuperblocks. A superblock (“SB”) includes luma sample values organizedas a luma block and corresponding chroma sample values organized aschroma blocks. A root SB with size 128×128 can be recursivelypartitioned into smaller square SBs of size 64×64, 32×32, 16×16, or 8×8.A given square 2N×2N SB can also be partitioned into two rectangularN×2N or 2N×N SBs, in which case the smaller N×2N or 2N×N SBs are notfurther partitioned. Thus, the size of a SB can be 128×128, 128×64,64×128, 64×64, 64×32, 32×64, 32×32, 32×16, 16×32, 16×16, 16×8, 8×16, or8×8. Further, an 8×8 SB can be split into two 8×4 SBs, two 4×8 SBs, orfour 4×4 SBs for some operations.

Generally, prediction operations are performed for a SB as a predictionunit. A SB may be split into smaller blocks for transform operations, ormultiple SBs may be combined for a transform that covers multipleprediction units (SBs). Parameters such as prediction mode (inter orintra), motion vector (“MV”) data, reference frame data, interpolationfilter type, transform size and type, skip status, and segment index aretypically specified for a SB. For a small SB (e.g., 8×4 SB, 4×8 SB, or4×4 SB), however, some parameters (such as prediction mode and MV data)can be signaled for the small SB while other parameters are signaled forthe 8×8 SB that includes the small SB.

In implementations of encoding for extensions of the H.265 standard, forexample, the encoder (400) splits the content of a picture (or slice ortile) into coding tree units. A coding tree unit (“CTU”) includes lumasample values organized as a luma coding tree block (“CTB”) andcorresponding chroma sample values organized as two chroma CTBs. Thesize of a CTU (and its CTBs) is selected by the encoder. A luma CTB cancontain, for example, 64×64, 32×32 or 16×16 luma sample values. A CTUincludes one or more coding units. A coding unit (“CU”) has a lumacoding block (“CB”) and two corresponding chroma CBs. A CTU can be splitinto four CUs, with each CU possibly being split further into smallerCUs. The smallest allowable size of CU (e.g., 8×8, 16×16) can besignaled in the bitstream.

Generally, a CU has a prediction mode such as inter or intra. A CUincludes one or more prediction units for purposes of signaling ofprediction information (such as prediction mode details, displacementvalues, etc.) and/or prediction processing. A prediction unit (“PU”) hasa luma prediction block (“PB”) and two chroma PBs. According to theH.265 standard, for an intra-predicted CU, the PU has the same size asthe CU, unless the CU has the smallest size (e.g., 8×8). In that case,the CU can be split into smaller PUs (e.g., four 4×4 PUs, two 4×8 PUs,or two 8×4 PUs, if the smallest CU size is 8×8, for intra-pictureprediction) or the PU can have the smallest CU size, as indicated by asyntax element for the CU. Alternatively, a larger CU can be split intomultiple PUs. A CU also has one or more transform units for purposes ofresidual coding/decoding, where a transform unit (“TU”) has a lumatransform block (“TB”) and two chroma TBs. A PU in an intra-predicted CUmay contain a single TU (equal in size to the PU) or multiple TUs. Theencoder decides how to partition video into CTUs, CUs, PUs, TUs, etc.

As used herein, the term “block” can indicate an m×n arrangement ofsample values, a residual data unit, a CTB, a CB, a PB, a TB, or someother set of sample values, depending on context. The term “unit” canindicate a SB, a macroblock, a CTU, a CU, a PU, a TU, or some other setof blocks, or it can indicate a single block, depending on context. Aluma block is an example of a primary component block for a YUV colorspace. The label “luma block” is sometimes used, however, to indicate aprimary component block even for another color space such as an RGBcolor space, BGR color space, or GBR color space. Similarly, a chromablock is an example of a secondary component block for a YUV colorspace. The label “chroma block” is sometimes used, however, to indicatea secondary component block even for another color space such as an RGBcolor space, BGR color space, or GBR color space.

With reference to FIG. 4a , the general encoding control (420) receivespictures for the input video signal (405) as well as feedback (notshown) from various modules of the encoder (400). Overall, the generalencoding control (420) provides control signals (not shown) to othermodules (such as the tiling module (410), transformer/scaler/quantizer(430), scaler/inverse transformer (435), intra-picture estimator (440),intra-picture predictor (445), motion estimator (450), motioncompensator (455) and intra/inter switch) to set and change codingparameters during encoding. In particular, the general encoding control(420) can decide whether and how to use intra BC prediction duringencoding. The general encoding control (420) can also evaluateintermediate results during encoding, for example, performingrate-distortion analysis. The general encoding control (420) producesgeneral control data (422) that indicates decisions made duringencoding, so that a corresponding decoder can make consistent decisions.The general control data (422) is provided to the headerformatter/entropy coder (490).

If the current picture is predicted using inter-picture prediction, amotion estimator (450) estimates the motion of blocks of sample valuesof a current picture of the input video signal (405) with respect to oneor more reference pictures. For example, the motion estimator (450)estimates the motion of a current block of the current picture relativeto one or more reference pictures. For motion estimation andcompensation, a reference block is a block of sample values in areference picture that is used to generate prediction values for thecurrent block of sample values of the current picture. The decodedpicture buffer (470) buffers one or more reconstructed previously codedpictures for use as reference pictures. When multiple reference picturesare used, the multiple reference pictures can be from different temporaldirections or the same temporal direction. The motion estimator (450)produces as side information motion data (452) such as MV data, mergemode index values or other MV selection data, and reference pictureselection data. The motion data (452) is provided to the headerformatter/entropy coder (490) as well as the motion compensator (455).

The motion compensator (455) applies MVs to the reconstructed referencepicture(s) from the decoded picture buffer (470). The motion compensator(455) produces motion-compensated predictions for blocks of the currentpicture.

In a separate path within the encoder (400), an intra-picture estimator(440) determines how to perform intra-picture prediction for blocks ofsample values of a current picture of the input video signal (405) usingother, previously reconstructed sample values in the current picture.The current picture can be entirely or partially coded usingintra-picture coding. Using sample values of a reconstruction (438) ofthe current picture, for intra spatial prediction (extrapolation), theintra-picture estimator (440) determines how to spatially predict samplevalues of a current block of the current picture from neighboring,previously reconstructed sample values of the current picture.

Or, for intra BC prediction, the intra-picture estimator (440) estimatesdisplacement from a current block of the current picture to a positionin the other, previously reconstructed sample values of the currentpicture. For intra BC prediction, a reference block of sample values inthe current picture is used to generate prediction values for thecurrent block. For example, for intra BC prediction, the intra-pictureestimator (440) estimates displacement from a current block to areference block, which can be indicated with a BV value. When theintra-picture estimator (440) determines BV values for intra BCprediction, the intra-picture estimator (440) can enforce constraints onlocations of reference blocks, as explained below.

Depending on implementation, the intra-picture estimator (440) canperform BV estimation for the current block using reconstructed samplevalues before in-loop filtering, using reconstructed sample values afterin-loop filtering, or using input sample values. In general, by usingunfiltered, reconstructed sample values (or input sample values) for BVestimation, the intra-picture estimator (440) can avoid asequential-processing bottleneck (which may result from filteringreconstructed sample values before BV estimation/intra BC prediction).On the other hand, storing unfiltered, reconstructed sample values forBV estimation uses additional memory. Additional details about optionsfor in-loop filtering are provided below.

The intra-picture estimator (440) produces as side information intraprediction data (442), such as information indicating whether intraprediction uses spatial prediction or intra BC prediction, predictionmode direction (for intra spatial prediction), and BV values (for intraBC prediction). The intra prediction data (442) is provided to theheader formatter/entropy coder (490) as well as the intra-picturepredictor (445).

According to the intra prediction data (442), the intra-picturepredictor (445) spatially predicts sample values of a current block ofthe current picture from neighboring, previously reconstructed samplevalues of the current picture. Or, for intra BC prediction, theintra-picture predictor (445) predicts the sample values of a currentblock using previously reconstructed sample values of a reference block,which is indicated by a displacement (BV value) for the current block.

In some example implementations, intra BC prediction is a special caseof motion compensation for which the reference picture is the currentpicture. In such implementations, functionality described above withreference to the intra-picture estimator (440) and intra-picturepredictor (445) for BV estimation and intra BC prediction can beimplemented in the motion estimator (450) and motion compensator (455),respectively. The motion estimator (450) can enforce constraints onlocations of reference blocks, as explained below.

For a palette coding mode, the encoder (400) represents at least some ofthe sample values of a unit using a palette. The palette representscolors used in the unit. For example, the palette maps index values 0,1, 2, . . . , p to corresponding colors, which can be in RGB 4:4:4format, BGR 4:4:4 format, GBR 4:4:4 format, YUV 4:4:4 format, or anotherformat (color space, color sampling rate). An index value can representa RGB triplet, BGR triplet or GBR triplet for a pixel, where a pixel isa set of co-located sample values. For encoding of the unit, appropriateindex values replace the sample values of pixels in the unit. A rarevalue in the unit can be encoded using an escape code value and literalvalues, instead of using an index value in the palette. The palette canchange from unit to unit, and palette data specifying the palettes canbe signaled in the bitstream.

The intra/inter switch selects whether the prediction (458) for a givenblock will be a motion-compensated prediction or intra-pictureprediction.

In some example implementations, no residual is calculated for a unitencoded in palette coding mode or intra BC prediction mode. Instead,residual coding is skipped, and the predicted sample values are used asthe reconstructed sample values. Alternatively, residual coding can beperformed for a unit encoded in intra BC prediction mode.

When residual coding is not skipped, the difference (if any) between ablock of the prediction (458) and a corresponding part of the originalcurrent picture of the input video signal (405) provides values of theresidual (418). During reconstruction of the current picture, whenresidual values have been encoded/signaled, reconstructed residualvalues are combined with the prediction (458) to produce an approximateor exact reconstruction (438) of the original content from the videosignal (405). (In lossy compression, some information is lost from thevideo signal (405).)

As part of residual coding, in the transformer/scaler/quantizer (430),when a frequency transform is not skipped, a frequency transformerconverts spatial-domain video information into frequency-domain (i.e.,spectral, transform) data. For block-based video coding, the frequencytransformer applies a discrete cosine transform (“DCT”), an integerapproximation thereof, or another type of forward block transform (e.g.,a discrete sine transform or an integer approximation thereof) to blocksof prediction residual data (or sample value data if the prediction(458) is null), producing blocks of frequency transform coefficients.The transformer/scaler/quantizer (430) can apply a transform withvariable block sizes. In this case, the transformer/scaler/quantizer(430) can determine which block sizes of transforms to use for theresidual values for a current block. The scaler/quantizer scales andquantizes the transform coefficients. The encoder (400) can set valuesfor quantization parameter (“QP”) for a picture, tile, slice and/orother portion of video, and quantize transform coefficients accordingly.For example, the quantizer applies dead-zone scalar quantization to thefrequency-domain data with a quantization step size that varies on apicture-by-picture basis, tile-by-tile basis, slice-by-slice basis,block-by-block basis, frequency-specific basis, or other basis. Thequantized transform coefficient data (432) is provided to the headerformatter/entropy coder (490). If the frequency transform is skipped,the scaler/quantizer can scale and quantize the blocks of predictionresidual data (or sample value data if the prediction (458) is null),producing quantized values that are provided to the headerformatter/entropy coder (490).

To reconstruct residual values, in the scaler/inverse transformer (435),a scaler/inverse quantizer performs inverse scaling and inversequantization on the quantized transform coefficients. When the transformstage has not been skipped, an inverse frequency transformer performs aninverse frequency transform, producing blocks of reconstructedprediction residual values or sample values. If the transform stage hasbeen skipped, the inverse frequency transform is also skipped. In thiscase, the scaler/inverse quantizer can perform inverse scaling andinverse quantization on blocks of prediction residual data (or samplevalue data), producing reconstructed values. When residual values havebeen encoded/signaled, the encoder (400) combines reconstructed residualvalues with values of the prediction (458) (e.g., motion-compensatedprediction values, intra-picture prediction values) to form thereconstruction (438). When residual values have not beenencoded/signaled, the encoder (400) uses the values of the prediction(458) as the reconstruction (438).

For intra-picture prediction, the values of the reconstruction (438) canbe fed back to the intra-picture estimator (440) and intra-picturepredictor (445). The values of the reconstruction (438) can be used formotion-compensated prediction of subsequent pictures.

The values of the reconstruction (438) can be further filtered. Afiltering control (460) determines how to perform adaptive deblockfiltering, sample adaptive offset (“SAO”) filtering, and/or otherfiltering on values of the reconstruction (438), for a given picture ofthe video signal (405), within the motion compensation loop (that is,“in-loop” filtering). The filtering control (460) produces filtercontrol data (462), which is provided to the header formatter/entropycoder (490) and merger/filter(s) (465).

In the merger/filter(s) (465), the encoder (400) merges content fromdifferent units (and tiles) into a reconstructed version of the picture.The encoder (400) selectively performs deblock filtering, SAO filtering,and/or other filtering (such as constrained directional enhancementfiltering or loop restoration filtering) according to the filter controldata (462) and rules for filter adaptation, so as to adaptively smoothdiscontinuities across boundaries in the pictures. Filtering such asde-ringing filtering or adaptive loop filtering (not shown) canalternatively or additionally be applied. Tile boundaries can beselectively filtered or not filtered at all, depending on settings ofthe encoder (400), and the encoder (400) may provide syntax elementswithin the coded bitstream to indicate whether or not such filtering wasapplied. The decoded picture buffer (470) buffers the reconstructedcurrent picture for use in subsequent motion-compensated prediction.

The timing of in-loop filtering operations can increase memory cost orcomputational complexity of encoding when intra BC prediction is used.If intra BC prediction uses reconstructed sample values in the currentpicture after filtering, there may be delay while in-loop filteringoperations are applied. On the other hand, if intra BC prediction usesreconstructed sample values in the current picture before in-loopfiltering, an additional picture buffer may be needed to storeunfiltered, reconstructed sample values for intra BC prediction (inaddition to the picture buffer that stores filtered, reconstructedsample values). Also, if in-loop filtering is applied prior to BVestimation, there may be a region of filter support that overlaps acurrent block, which complicates memory access patterns. In some exampleimplementations, in-loop filtering operations are disabled for a picturewhen intra BC prediction is enabled for that picture. With thisconstraint, an additional picture buffer is not needed—the singlepicture buffer stores unfiltered, reconstructed sample values.Alternatively, the encoder (400) can apply some in-loop filteringoperations before BV estimation/intra BC prediction, and performadditional or alternative filtering in a later processing stage.

The header formatter/entropy coder (490) formats and/or entropy codesthe general control data (422) (e.g., mode decisions), quantizedtransform coefficient data (432), intra prediction data (442) (e.g., BVvalues), motion data (452), and filter control data (462). For themotion data (452), the header formatter/entropy coder (490) can selectand entropy code merge mode index values, or a default MV predictor canbe used. In some cases, the header formatter/entropy coder (490) alsodetermines MV differentials for MV values (relative to MV predictors forthe MV values), then entropy codes the MV differentials. For the intraprediction data (442), a BV value can be encoded using prediction. Theprediction can use a default predictor (e.g., a BV value from aneighboring unit, or median of BV values from multiple neighboringunits). When multiple predictors are possible, a predictor index canindicate which of the multiple predictors to use for prediction of theBV value. The header formatter/entropy coder (490) can select andentropy code predictor index values (for intra BC prediction), or adefault predictor can be used. In some cases, the headerformatter/entropy coder (490) also determines differentials (relative topredictors for the BV values), then entropy codes the differentials. Forpalette coding mode, the header formatter/entropy coder (490) can encodepalette data.

The header formatter/entropy coder (490) can perform entropy coding invarious ways. Typical entropy coding techniques includeExponential-Golomb coding, Golomb-Rice coding, context-adaptive binaryarithmetic coding (“CABAC”), differential coding, Huffman coding, runlength coding, variable-length-to-variable-length (“V2V”) coding,variable-length-to-fixed-length (“V2F”) coding, Lempel-Ziv (“LZ”)coding, dictionary coding, probability interval partitioning entropycoding (“PIPE”), and combinations of the above. The headerformatter/entropy coder (490) can use different coding techniques fordifferent kinds of data, can apply multiple techniques in combination(e.g., by applying Golomb-Rice coding followed by CABAC), and can choosefrom among multiple code tables or contexts within a particular codingtechnique.

The header formatter/entropy coder (490) provides the encoded data inthe coded video bitstream (495). The codec format of the coded videobitstream (495) can be an extension of AV1 format, VPx format (e.g.,VP8, VP9), H.26x format (e.g., H.261, H.262, H.263, H.264, H.265),Windows Media Video format, VC-1 format, MPEG-x format (e.g., MPEG-1,MPEG-2, or MPEG-4), or another codec format.

Depending on implementation and the type of compression desired, modulesof an encoder (400) can be added, omitted, split into multiple modules,combined with other modules, and/or replaced with like modules. Inalternative embodiments, encoders with different modules and/or otherconfigurations of modules perform one or more of the describedtechniques. Specific embodiments of encoders typically use a variationor supplemented version of the encoder (400). The relationships shownbetween modules within the encoder (400) indicate general flows ofinformation in the encoder; other relationships are not shown for thesake of simplicity.

With reference to FIG. 3, in addition to producing encoded data for acoded picture (341), the encoder (340) produces memory managementcontrol operation (“MMCO”) signals (342) or reference picture set(“RPS”) information. The RPS is the set of pictures that may be used forreference in motion compensation for a current picture or any subsequentpicture. If the current picture is not the first picture that has beenencoded, when performing its encoding process, the encoder (340) may useone or more previously encoded/decoded pictures (369) that have beenstored in a decoded picture temporary memory storage area (360), whichis an example of decoded picture buffer (470). Such stored decodedpictures (369) are used as reference pictures for inter-pictureprediction of the content of the current source picture (331). TheMMCO/RPS information (342) indicates to a decoder which reconstructedpictures may be used as reference pictures, and hence should be storedin a picture storage area.

The coded pictures (341) and MMCO/RPS information (342) (or informationequivalent to the MMCO/RPS information (342), since the dependencies andordering structures for pictures are already known at the encoder (340))are processed by a decoding process emulator (350) in the encoder system(300) of FIG. 3. The decoding process emulator (350) implements some ofthe functionality of a decoder, for example, decoding tasks toreconstruct sample values of the current picture and reference pictures.(In practice, the decoding process emulator (350) is implemented as partof the encoder (340). For example, the decoding process emulator (350)includes the scaler and inverse transformer (435), the merger/filters(465) and other functionality to reconstruct sample values.) In a mannerconsistent with the MMCO/RPS information (342), the decoding processesemulator (350) determines whether a given coded picture (341) needs tobe reconstructed and stored for use as a reference picture ininter-picture prediction of subsequent pictures to be encoded. If acoded picture (341) needs to be stored, the decoding process emulator(350) models the decoding process that would be conducted by a decoderthat receives the coded picture (341) and produces a correspondingdecoded picture (351). In doing so, when the encoder (340) has useddecoded picture(s) (369) that have been stored in the decoded picturestorage area (360), the decoding process emulator (350) also uses thedecoded picture(s) (369) from the storage area (360) as part of thedecoding process.

The decoded picture temporary memory storage area (360) includesmultiple picture buffer storage areas (361, 362, . . . , 36 n). In amanner consistent with the MMCO/RPS information (342), the decodingprocess emulator (350) manages the contents of the storage area (360) inorder to identify any picture buffers (361, 362, etc.) with picturesthat are no longer needed by the encoder (340) for use as referencepictures. After modeling the decoding process, the decoding processemulator (350) stores a newly decoded picture (351) in a picture buffer(361, 362, etc.) that has been identified in this manner.

The coded pictures (341) and MMCO/RPS information (342) are buffered ina temporary coded data area (370). The coded data that is aggregated inthe coded data area (370) contains, as part of the syntax of anelementary coded video bitstream, encoded data for one or more picturesrepresented with syntax elements for various layers of bitstream syntax.The coded data that is aggregated in the coded data area (370) can alsoinclude media metadata relating to the coded video data (e.g., as one ormore parameters in one or more supplemental enhancement information(“SEI”) messages or video usability information (“VUI”) messages).

The aggregated data (371) from the temporary coded data area (370) isprocessed by a channel encoder (380). The channel encoder (380) canpacketize and/or multiplex the aggregated data for transmission orstorage as a media stream (e.g., according to a media program stream ortransport stream format), in which case the channel encoder (380) canadd syntax elements as part of the syntax of the media transmissionstream. Or, the channel encoder (380) can organize the aggregated datafor storage as a file (e.g., according to a media container format), inwhich case the channel encoder (380) can add syntax elements as part ofthe syntax of the media storage file. Or, more generally, the channelencoder (380) can implement one or more media system multiplexingprotocols or transport protocols, in which case the channel encoder(380) can add syntax elements as part of the syntax of the protocol(s).The channel encoder (380) provides output to a channel (390), whichrepresents storage, a communications connection, or another channel forthe output. The channel encoder (380) or channel (390) may also includeother elements (not shown), e.g., for forward-error correction (“FEC”)encoding and analog signal modulation.

IV. Example Decoder Systems and Video Decoders.

FIG. 5 is a block diagram of an example decoder system (500) inconjunction with which some described embodiments may be implemented.The decoder system (500) can be a general-purpose decoding tool capableof operating in any of multiple decoding modes such as a low-latencydecoding mode for real-time communication and a higher-latency decodingmode for media playback from a file or stream, or it can be aspecial-purpose decoding tool adapted for one such decoding mode. Thedecoder system (500) can be implemented as part of an operating systemmodule, as part of an application library, as part of a standaloneapplication, using GPU hardware, or using special-purpose hardware.Overall, the decoder system (500) receives coded data from a channel(510) and produces reconstructed pictures as output for an outputdestination (590). The received encoded data can include content encodedusing intra BC prediction with constraints on locations of referenceblocks as described herein.

The decoder system (500) includes a channel (510), which can representstorage, a communications connection, or another channel for coded dataas input. The channel (510) produces coded data that has been channelcoded. A channel decoder (520) can process the coded data. For example,the channel decoder (520) de-packetizes and/or demultiplexes data thathas been aggregated for transmission or storage as a media stream (e.g.,according to a media program stream or transport stream format), inwhich case the channel decoder (520) can parse syntax elements added aspart of the syntax of the media transmission stream. Or, the channeldecoder (520) separates coded video data that has been aggregated forstorage as a file (e.g., according to a media container format), inwhich case the channel decoder (520) can parse syntax elements added aspart of the syntax of the media storage file. Or, more generally, thechannel decoder (520) can implement one or more media systemdemultiplexing protocols or transport protocols, in which case thechannel decoder (520) can parse syntax elements added as part of thesyntax of the protocol(s). The channel (510) or channel decoder (520)may also include other elements (not shown), e.g., for FEC decoding andanalog signal demodulation.

The coded data (521) that is output from the channel decoder (520) isstored in a temporary coded data area (530) until a sufficient quantityof such data has been received. The coded data (521) includes codedpictures (531) and MMCO/RPS information (532). The coded data (521) inthe coded data area (530) contain, as part of the syntax of anelementary coded video bitstream, coded data for one or more pictures.The coded data (521) in the coded data area (530) can also include mediametadata relating to the encoded video data (e.g., as one or moreparameters in one or more SEI messages or VUI messages).

In general, the coded data area (530) temporarily stores coded data(521) until such coded data (521) is used by the decoder (550). At thatpoint, coded data for a coded picture (531) and MMCO/RPS information(532) are transferred from the coded data area (530) to the decoder(550). As decoding continues, new coded data is added to the coded dataarea (530) and the oldest coded data remaining in the coded data area(530) is transferred to the decoder (550).

The decoder (550) decodes coded data (521) for a coded picture (531) toproduce a corresponding decoded picture (551). FIG. 6 is a block diagramof a generalized decoder (600) in conjunction with which some describedembodiments may be implemented. The decoder (600) receives encoded data,for a coded picture (531), in a coded video bitstream (605) and producesreconstructed video (695) for output as the decoded picture (551). Thecodec format of the coded video bitstream (605) can be an extension ofAV1 format, VPx format (e.g., VP8, VP9), H.26x format (e.g., H.261,H.262, H.263, H.264, H.265), Windows Media Video format, VC-1 format,MPEG-x format (e.g., MPEG-1, MPEG-2, or MPEG-4), or another codecformat.

A picture can be organized as multiple tiles of the same size ordifferent sizes. In implementations that extend the AV1 format, apicture can also be organized as one or more segments. Inimplementations that extend the H.265 format, a picture can also beorganized as one or more slices. The content of a picture (or tile orslice) is further organized as blocks of sample values. The decoder(600) is block-based and uses a block format that depends onimplementation. Blocks may be further sub-divided at different stages.For example, a picture can be divided into 256×256 blocks, 128×128blocks, 64×64 blocks, 32×32 blocks or 16×16 blocks, which can in turn bedivided into smaller blocks of sample values. In implementations ofdecoding for extensions of the AV1 format, a picture is partitioned intoroot SBs, as described above, which can be recursively partitioned intosmaller SBs. In implementations of decoding for the H.265 standard, apicture is partitioned into CTUs (CTBs), CUs (CBs), PUs (PBs), and TUs(TBs), as described above.

The decoder (600) decompresses pictures using intra-picture decoding andinter-picture decoding. Many of the components of the decoder (600) areused for both intra-picture decoding and inter-picture decoding. Theexact operations performed by those components can vary depending on thecodec format and type of information being decompressed.

A buffer receives encoded data in the coded video bitstream (605) andmakes the received encoded data available to the parser/entropy decoder(610). The parser/entropy decoder (610) entropy decodes entropy-codeddata, typically applying the inverse of entropy coding performed in theencoder (500) (e.g., context-adaptive binary arithmetic decoding). As aresult of parsing and entropy decoding, the parser/entropy decoder (610)produces general control data (622), quantized transform coefficientdata (632), intra prediction data (642), motion data (652) and filtercontrol data (662). For the intra prediction data (642), if predictorindex values are signaled, the parser/entropy decoder (610) can entropydecode the predictor index values, e.g., using context-adaptive binaryarithmetic decoding. In some cases, the parser/entropy decoder (610)also entropy decodes differentials for BV values (e.g., usingcontext-adaptive binary arithmetic decoding), then combines thedifferentials with corresponding predictors to reconstruct the BVvalues. In other cases, the differential is omitted from the bitstream,and the BV value is simply the predictor (e.g., indicated with thepredictor index value). For palette decoding mode, the parser/entropydecoder (610) can decode palette data.

The general decoding control (620) receives the general control data(622) and provides control signals (not shown) to other modules (such asthe scaler/inverse transformer (635), intra-picture predictor (645),motion compensator (655) and intra/inter switch) to set and changedecoding parameters during decoding.

If the current picture is predicted using inter-picture prediction, amotion compensator (655) receives the motion data (652), such as MVdata, reference picture selection data, and merge mode index values orother MV selection data. The motion compensator (655) applies MVs to thereconstructed reference picture(s) from the decoded picture buffer(670). The motion compensator (655) produces motion-compensatedpredictions for inter-coded blocks of the current picture. The decodedpicture buffer (670) stores one or more previously reconstructedpictures for use as reference pictures. (In FIG. 5, the decoded picturetemporary memory storage area (560) is an example of decoded picturebuffer. The decoded picture temporary memory storage area (560) includesmultiple picture buffer storage areas (561, 562, . . . , 56 n), fromwhich the decoder (550) reads reconstructed sample values of previouslydecoded pictures (569).)

In a separate path within the decoder (600), the intra-picture predictor(645) receives the intra prediction data (642), such as informationindicating whether intra-picture prediction uses spatial prediction orintra BC prediction, as well as prediction mode direction (for intraspatial prediction) or BV values (for intra BC prediction). For intraspatial prediction, using values of a reconstruction (638) of thecurrent picture, according to prediction mode data, the intra-picturepredictor (645) spatially predicts sample values of a current block ofthe current picture from neighboring, previously reconstructed samplevalues of the current picture. Or, for intra BC prediction, theintra-picture predictor (645) predicts the sample values of a currentblock using previously reconstructed sample values of a reference block(in the current picture), which is indicated by a displacement (BVvalue) for the current block. The BV values are consistent withconstraints on locations of reference blocks, as explained below.

In some example implementations, intra BC prediction is a special caseof motion compensation for which the reference picture is the currentpicture. In such implementations, functionality described above withreference to the intra-picture predictor (645) for intra BC predictioncan be implemented in the motion compensator (655).

For a palette decoding mode, the decoder (600) uses a palette thatrepresents at least some of the sample values of a unit. The palettemaps index values to corresponding colors used in the unit. For example,the palette maps index values 0, 1, 2, . . . , p to correspondingcolors, which can be in RGB 4:4:4 format, BGR 4:4:4 format, GBR 4:4:4format, YUV 4:4:4 format, or another format (color space, color samplingrate). An index value can represent a RGB triplet, BGR triplet or GBRtriplet for a pixel. During decoding, for positions in the unit, indexvalues from the palette are replaced with the appropriate sample values.An escape-coded value in the unit can be decoded using an escape codevalue and literal values. The palette can change from unit to unit,based on palette data signaled in the bitstream.

The intra/inter switch selects values of a motion-compensated predictionor intra-picture prediction for use as the prediction (658) for a givenblock. For example, the intra/inter switch can be controlled based on asyntax element encoded for a unit of a picture that can containintra-predicted units and inter-predicted units.

When residual values have been encoded/signaled, the decoder (600)combines the prediction (658) with reconstructed residual values toproduce the reconstruction (638) of the content from the video signal.When residual values have not been encoded/signaled, the decoder (600)uses the values of the prediction (658) as the reconstruction (638). Aspreviously noted, in some example implementations, no residual iscalculated for a unit encoded in palette coding mode or intra BCprediction mode. Instead, residual coding is skipped, and the predictedsample values are used as the reconstructed sample values.Alternatively, residual coding can be performed for a unit encoded inintra BC prediction mode.

To reconstruct the residual when residual values have beenencoded/signaled, the scaler/inverse transformer (635) receives andprocesses the quantized transform coefficient data (632). In thescaler/inverse transformer (635), a scaler/inverse quantizer performsinverse scaling and inverse quantization on the quantized transformcoefficients. For example, the scaler/inverse quantizer sets values forQP for a picture, tile, slice and/or other portion of video based onsyntax elements in the bitstream, and inverse quantizes transformcoefficients accordingly. An inverse frequency transformer performs aninverse frequency transform, producing blocks of reconstructedprediction residual values or sample values. The inverse frequencytransform can be an inverse DCT, an integer approximation thereof, oranother type of inverse frequency transform (e.g., an inverse discretesine transform or an integer approximation thereof). If the frequencytransform was skipped during encoding, the inverse frequency transformis also skipped. In this case, the scaler/inverse quantizer can performinverse scaling and inverse quantization on blocks of predictionresidual data (or sample value data), producing reconstructed values.

For intra-picture prediction, the values of the reconstruction (638) canbe fed back to the intra-picture predictor (645). For inter-pictureprediction, the values of the reconstruction (638) can be furtherfiltered. In the merger/filter(s) (665), the decoder (600) mergescontent from different units into a reconstructed version of thepicture. The decoder (600) selectively performs adaptive deblockfiltering, SAO filtering, and/or other filtering (such as constraineddirectional enhancement filtering and loop restoration filtering) withinthe motion compensation loop according to the filter control data (662)and rules for filter adaptation, so as to adaptively smoothdiscontinuities across boundaries in the pictures. Filtering such asde-ringing filtering or adaptive loop filtering (not shown) canalternatively or additionally be applied. Tile boundaries can beselectively filtered or not filtered at all, depending on settings ofthe decoder (600) or a syntax element within the encoded bitstream data.The decoded picture buffer (670) buffers the reconstructed currentpicture for use in subsequent motion-compensated prediction. (Withreference to FIG. 5, the decoder (550) can use the MMCO/RPS information(532) to identify a picture buffer (561, 562, etc.) in which it canstore a decoded picture (551). The decoder (550) stores the decodedpicture (551) in that picture buffer.)

As explained in section III, the timing of in-loop filtering operationscan increase memory cost or computational complexity of decoding whenintra BC prediction is used. If intra BC prediction uses reconstructedsample values in the current picture after filtering, there may be delaywhile in-loop filtering operations are applied. On the other hand, ifintra BC prediction uses reconstructed sample values in the currentpicture before in-loop filtering, an additional picture buffer may beneeded to store unfiltered, reconstructed sample values for intra BCprediction (in addition to the picture buffer that stores filtered,reconstructed sample values). In some example implementations, in-loopfiltering operations are disabled for a picture when intra BC predictionis enabled for that picture. With this constraint, an additional picturebuffer for unfiltered, reconstructed sample values is not needed—thesingle picture buffer stores unfiltered, reconstructed sample values.Alternatively, the decoder (600) can apply some in-loop filteringoperations before intra BC prediction, and perform additional oralternative filtering in a later processing stage.

The decoder (600) can also include a post-processing filter. Thepost-processing filter can perform deblock filtering, de-ringingfiltering, adaptive Wiener filtering, film-grain reproduction filtering,SAO filtering or another kind of filtering. Whereas “in-loop” filteringis performed on reconstructed sample values of pictures in a motioncompensation loop, and hence affects sample values of referencepictures, the post-processing filter is applied to reconstructed samplevalues outside of the motion compensation loop, before output fordisplay.

Depending on implementation and the type of decompression desired,modules of the decoder (600) can be added, omitted, split into multiplemodules, combined with other modules, and/or replaced with like modules.In alternative embodiments, decoders with different modules and/or otherconfigurations of modules perform one or more of the describedtechniques. Specific embodiments of decoders typically use a variationor supplemented version of the decoder (600). The relationships shownbetween modules within the decoder (600) indicate general flows ofinformation in the decoder; other relationships are not shown for thesake of simplicity.

With reference to FIG. 5, an output sequencer (580) identifies when thenext picture to be produced in output order is available in the decodedpicture storage area (560). When the next picture (581) to be producedin output order is available in the decoded picture storage area (560),it is read by the output sequencer (580) and output to the outputdestination (590) (e.g., display). In general, the order in whichpictures are output from the decoded picture storage area (560) by theoutput sequencer (580) may differ from the order in which the picturesare decoded by the decoder (550).

V. Constraints on Locations of Reference Blocks for Intra BC Prediction.

This section presents examples of various constraints on locations ofreference blocks for intra block copy (“BC”) prediction.

A. Intra BC Prediction—Introduction.

In general, intra BC prediction mode uses intra-picture prediction inwhich sample values of a current block of a picture are predicted usingreconstructed sample values in the same picture. A block vector (“BY”)value indicates a displacement from the current block to a block of thepicture (the “reference block”) that includes the reconstructed samplevalues used for prediction. The reference block provides predictedvalues for the current block. The sample values used for prediction arepreviously reconstructed sample values, which are thus available at theencoder during encoding and available at the decoder during decoding.The BV value is signaled in the bitstream, and a decoder can use the BVvalue to determine the reference block of the picture to use forprediction.

FIG. 7 shows an example (700) of intra BC prediction for a current block(730) of a current picture (710). The current block can be a block of aroot SB, a block of a macroblock (“MB”), a prediction block (“PB”) of aprediction unit (“PU”), or other block. The block can be a primarycomponent block (e.g., luma block) or secondary component block (e.g.,chroma block). The size of the current block can be 128×128, 128×64,64×128, 64×64, 64×32, 32×64, 32×32, 32×16, 16×32, 16×16, 16×8, 8×16,8×8, 8×4, 4×8, 4×4 or some other size. Blocks can result from symmetricor asymmetric partitioning of a larger block into smaller blocks forpurposes of intra BC prediction. More generally, the size of the currentblock is m×n, where each of m and n is a whole number, and where m and ncan be equal to each other or can have different values. Thus, thecurrent block can be square or rectangular. Alternatively, the currentblock can have some other shape.

The BV value (740) indicates a displacement (or offset) from the currentblock (730) to a reference block (750), within the current picture(710), that includes the reconstructed sample values used forprediction. The sample values of the reference block (750) can beidentical to the sample values of the current block (730), or they canbe an approximation of the sample values of the current block (730).Suppose the top-left position of a current block is at position (x₀, y₀)in the current picture, and suppose the top-left position of thereference block is at position (x₁, y₁) in the current picture. The BVvalue indicates the displacement (x₁-x₀, y₁-y₀). For example, if thetop-left position of the current block is at position (256, 128), andthe top-left position of the reference block is at position (106, 104),the BV value is (−150, −24). In this example, a negative horizontaldisplacement indicates a position to the left of the current block, anda negative vertical displacement indicates a position above the currentblock.

FIG. 8 shows an example (800) illustrating some of the candidatereference blocks for a current block (830) of a current picture (810).Four BV values (841, 842, 843, 844) indicate displacements for fourcandidate reference blocks. A candidate reference block can overlap withother candidate reference blocks, as shown for the candidate referenceblocks indicated by the BV values (843, 844). In FIG. 8, the search areafor candidate reference blocks for the current block (830) covers all ofthe previously reconstructed content of the current picture (810). Inpractice, various constraints can limit the locations for candidatereference blocks, as described below.

B. Example Syntax, Range, and Precision for BV Values.

In general, the BV value for a block can be signaled at unit level, inwhich case other blocks in the unit use the same BV value or a scaledversion thereof. Alternatively, different BV values can be found fordifferent blocks of a unit. A BV value “associated with” a unit can be aBV value for all blocks of that unit or a BV value for a single block ofthat unit.

A BV value associated with a unit can be signaled in various ways. Forexample, a flag in the bitstream indicates whether or not intra BCprediction is used for a unit. If intra BC prediction is used, thedifference between the BV value associated with the unit and a predictedBV value is signaled in the bitstream. The predicted BV value can bedetermined according to any of various rules (e.g., the predicted BVvalue is: (a) the BV value of the unit to the left of the current unit;(b) the median of the BV values of the units to the left, above, andabove-right of the current unit; (c) the median of the BV values of theunits to the left, above, and above-left of the current unit; or (d) aBV value selected from among multiple candidate BV values, as indicatedby an index value or other data signaled in the bitstream). Duringencoding, an encoder determines a BV difference value between the BVvalue associated with the unit and the predicted BV value, and theencoder signals the BV difference value in the bitstream. Duringcorresponding decoding, a decoder reconstructs the BV difference valueand combines the BV difference value with the predicted BV value.

The BV difference value can be signaled in any of various ways. Forexample, horizontal and vertical components of the BV difference valuecan be signaled jointly or separately. In some example implementations,a BV type syntax element indicates whether the horizontal and verticalcomponents, respectively, of the BV difference value are zero ornon-zero. If the horizontal component of the BV difference value isnon-zero, the horizontal component of the BV difference value issignaled in the bitstream. If the vertical component of the BVdifference value is non-zero, the vertical component of the BVdifference value is signaled in the bitstream. The BV type syntaxelement, horizontal component (if present), and vertical component (ifpresent) can be entropy coded.

To limit the bit rate cost associated with signaling of BV values, therange of BV values can be constrained. The range of BV values can be thesame horizontally and vertically, or range of BV values can be differenthorizontally and vertically. For example, the range is [4023, 1023] forthe horizontal component of a BV value, and the range is [−1023, 124]for the vertical component of a BV value. Alternatively, the horizontalcomponent and/or vertical component of a BV value can have other ranges.

When encoding screen content, most displacements for intra BC predictionor motion compensation are integer-sample displacements. In contrast,fractional-sample displacements are common in motion compensation whenencoding natural video. In some example implementations, the BV valueassociated with a unit has integer-sample value resolution for intra BCprediction. Alternatively, the BV value associated with a unit can havefractional-sample value resolution (e.g., ½-sample value resolution,¼-sample value resolution). In implementations in which intra BCprediction is a special case of motion compensation, the resolution ofMV/BV values can change adaptively (e.g., on a picture-by-picture basis,on a slice-by-slice basis, or on some other basis), depending on thetype of content being encoded.

In some example implementations, a BV value is signaled for a unit, andthe BV value is applied for all blocks of the unit. Depending on colorspace and color sampling rate, the BV value can be used for all blockswithout scaling, or the BV value can be scaled for blocks in differentcolor components. For example, if the format of the picture is YUV4:2:0, the BV value is scaled by a factor of two horizontally and scaledby a factor of two vertically. Or, if the format of the picture is YUV4:2:2, the BV value is scaled by a factor of two horizontally. On theother hand, if the format of the picture is YUV 4:4:4, RGB 4:4:4, BGR4:4:4, or GBR 4:4:4, the BV value used in intra BC prediction for aprimary component block is not scaled before use in intra BC predictionfor secondary component blocks.

C. Example BV Estimation Strategies.

Intra BC prediction can improve coding efficiency by exploitingredundancy (such as repeated patterns inside a picture) using BCoperations. Finding a matching reference block for a current block canbe computationally complex and time consuming, however, considering thenumber of candidate reference blocks that the encoder may evaluate. Tofind the BV value(s) to apply in intra BC prediction for a unit, anencoder can adopt various strategies.

For example, to determine the BV value associated with a current unit,an encoder can use exhaustive block matching—checking every candidatereference block for a current block using block-wise comparisons ofsample values. In most cases, an exhaustive block matching approach isnot feasible due to the computational complexity of the approach.

Instead, to determine the BV value associated with a current unit, anencoder can use block matching according to a search pattern. Theencoder checks candidate reference blocks along the search pattern,starting with candidate reference blocks that are likely to provide goodmatches, and stopping if a suitable candidate reference block is found.The criteria for what constitutes a “suitable” match can account forsample value differences (error) as well as bit rate associated withsignaling the associated BV value. Typically, the encoder continuessearching along the search pattern so long as candidate reference blockskeep providing a better match for the current block, but stops if thematch quality worsens. For example, the encoder follows a diamond searchpattern starting from a most likely candidate reference block (e.g., thecandidate reference block indicated by a predicted BV value), andcontinuing along successively larger diamond patterns that encircle thestarting location until a suitable match is found and a successivecandidate reference block along the search pattern provides a worsematch. Alternatively, an encoder can follow another search pattern.Block matching according to a search pattern is typically much fasterthan an exhaustive search, but an encoder may determine a BV value thatis “locally optimal” but not the best possible BV value.

As another option, to determine the BV value associated with a currentunit, an encoder can use hash-based block matching. Examples ofhash-based block matching are described in PCT Patent ApplicationPublication Nos. WO 2015/058395 and WO 2015/131325. When an encoder useshash-based block matching, the encoder determines a hash value for eachof multiple candidate reference blocks of a picture. The encoder alsodetermines a hash value for a current block. If two blocks areidentical, their hash values are the same. Using hash values, an encodercan quickly and efficiently identify candidate reference blocks thathave the same hash value as the current block, and filter out candidatereference blocks that have different hash values. The encoder can thenfurther evaluate those candidate reference blocks having the same hashvalue as the current block (e.g., by comparing sample values or bycomparing other hash values). Hash-based block matching usually speedsup the process of finding a matching block for a current block.

Calculating hash values for each possible block size can involveredundant computation. To reduce computational complexity, in someexample implementations, a hash dictionary (also called a hash table) isconstructed in a bottom-up manner, which can reduce the complexity ofgenerating the hash dictionary. For one 2N×2N block (for N=2, 4, 8, 16,32, or 64), a hash value is calculated based on the four N×N blocks ofthe 2N×2N block. This process continues recursively until 2×2 blocks(N=1) are reached. For a 2×2 block, the hash value is calculated fromluma sample values of the 2×2 block. The hash value of a 4×4 block isdetermined by applying a hash function to the hash values of the four2×2 blocks of the 4×4 block, and so on. Hash values for non-square blocksizes can be similarly calculated in a bottom-up manner.

The hash function for a block is, for example, a CRC function thatproduces 16-bit hash values or 24-bit hash values. Alternatively,another hash function can be used.

For 16-bit hash values, the number of possible hash values is 2¹⁶. For24-bit hash values, the number of possible hash values is 2²⁴. Even withmany possible hash values, there can be hash value collisions in whichtwo or more candidate reference blocks with different sample values havethe same hash value. To make hash value collisions less likely, the hashvalue range could be increased, but that would also increase the memorycost for the hash dictionary. Another option is to use a second hashfunction and inverted index table for an iterative, hierarchicalhash-based block matching approach. For example, hash values from afirst hash function (such as 16-bit hash values) for candidate referenceblocks are used to build an inverted index table. When multiplecandidate reference blocks have the same hash value from the first hashfunction, hash values from a second hash function (e.g., 24-bit hashvalues) for those candidate reference blocks are added to the invertedindex table. If hash values from the first hash function match for acurrent block and candidate reference block, hash values from the secondhash function for the current block and candidate reference block arecompared. If those hash values match, the candidate reference block isdeemed to match the current block.

For additional details about example data structures for a hashdictionary, example hash functions, example strategies for hash-basedblock matching, and optimizations (e.g., not computing hash values forsimple blocks), see PCT Patent Application Publication Nos. WO2015/058395 and WO 2015/131325.

As another option, to determine the BV value associated with a currentunit, an encoder can use a combination of hash-based block matching andblock matching according to a search pattern. In some exampleimplementations, an encoder constructs a hash dictionary for candidatereference blocks in a current picture using luma sample values (that is,input luma sample values) of the current picture. For a current block(current unit), the encoder first performs block matching according to adiamond search pattern, determining a BV value associated with thecurrent unit. Then, the encoder performs hash-based blockmatching—calculating a 16-bit hash value and a 24-bit hash value for theluma sample values of the current block, and checking the hashdictionary for matches between the current block and candidate referenceblocks. If there are multiple matches, the encoder can sort the matchesaccording to distance from the current block to the respective candidatereference blocks, choosing the nearest x matching blocks (where x is,e.g., 5). The encoder determines a BV value associated with the currentunit from the results of hash-based block matching. The encoder thenselects between the BV values determined from hash-based block matchingand block matching according to the diamond search pattern (e.g., viarate-distortion optimization).

Alternatively, the encoder can perform BV estimation in some other way.

D. Examples of Constraints on BV Values to Avoid Inefficient MemoryAccess Patterns in Hardware-Based Implementations of Intra BCPrediction.

A hardware-based encoder or hardware-based decoder typically has ahierarchical memory structure that includes “on-chip” memory and“off-chip” memory. The on-chip memory can be accessed quickly byprocessing cores, but is small due to cost considerations. For example,the on-chip memory is static memory such as one or more memory caches,which are fast to access but small. In typical encoding/decodingscenarios, on-chip memory stores some reconstructed sample values (e.g.,for a root SB or CTU being encoded/decoded) but is not large enough tostore reconstructed sample values for an entire picture beingencoded/decoded or available reference pictures. The processing corescan also access off-chip memory, which is much larger than the on-chipmemory but subject to rules for efficient access. For example, when acache between processing cores and off-chip memory is not practical,access to off-chip memory happens through bulk (batch)operations—writing or reading to a whole memory bank instead of a fewbytes. As a result, in typical encoding/decoding scenarios, there is adelay in writing reconstructed sample values for a current unit (e.g.,root SB, CTU) from on-chip memory back to a picture buffer in off-chipmemory. The picture buffer in off-chip memory can store reconstructedsample values for the entire picture being encoded/decoded. Otherpicture buffers in off-chip memory can store reference pictures.

In some previous approaches to intra BC prediction (e.g., in the H.265standard), a reference block can be located anywhere in previouslyreconstructed content in the current picture being encoded/decoded, evenright next to the current block. Thus, a reference block can be outsidea CTU that includes a current CU, inside the CTU, or even across theboundary of the CTU. In such approaches, intra BC prediction can be veryeffective in terms of lowering bit rate and improving quality, since anearby reference block may closely match the current block. In hardwareimplementations, however, memory access operations can be slow andinefficient when reconstructed sample values for one part of a referenceblock (close to the current block) are stored in on-chip memory butreconstructed sample values for the rest of the reference block (furtherfrom the current block) are separately stored in off-chip memory.

When determining the BV value associated with a current unit of apicture, an encoder can enforce various constraints on the location ofthe reference block indicated by the BV value, so as to avoidinefficient patterns of memory access during intra BC predictionoperations for a hardware-based encoder or hardware-based decoder. Thissection details examples of such constraints. The constraints, which areenforced by an encoder during BV estimation, can alternatively beexpressed in terms of bitstream conformance requirements for BV valuesin a codec format.

For example, in some previous approaches to intra BC prediction (e.g.,for some variations of the AV1 format) a reference block is constrainedto be outside the root SB that includes the current unit. Specifically,for a current unit that is part of a current root SB, whenencoding/decoding the current unit with intra BC prediction, a referenceblock is constrained to be outside the current root SB. FIG. 9a shows anexample (901) of such a constraint. The current picture beingencoded/decoded has multiple root SBs, including the current root SB(910) being encoded/decoded. The current block (920) is part of thecurrent root SB (910). A BV value (940) indicates the location of acandidate reference block in a search area (930). The search area (930)includes reconstructed content of previous root SBs of the currentpicture, but not the current root SB (910). The BV value (940) canindicate the location of a candidate reference block anywhere in thesearch area (930). A reference block cannot, however, be locatedanywhere in the not-yet-coded content of the current picture. Also, areference block cannot be located (even partially) within the currentroot SB (910), even for parts of the current root SB (910) that havealready been reconstructed. No part of a reference block can overlap thecurrent root SB (920). Thus, the current root SB (910) is off limits forintra BC prediction of the current block (920), even for previouslyreconstructed parts of the current root SB (910).

With such a constraint on location of the reference block, intra BCprediction is more “hardware friendly” since slow, inefficient memoryaccess operations for reference blocks split between on-chip memory andoff-chip memory can be avoided. The constraint on location of thereference block is consistent with the delay to batch reconstructedsample values of the current root SB for writing from on-chip memory tooff-chip memory. On the other hand, intra BC prediction may be lesseffective in terms of lowering bit rate and improving quality sincereference blocks nearest the current block might not be allowed. SinceSBs can be quite large (e.g., 128×128), the impact on compressionefficiency can be significant when nearby reference blocks are notallowed.

Even more restrictive constraints on the location of reference blockscan be enforced. For example, a reference block can be constrained to beoutside the root SB that includes the current unit and also outside theprevious root SB to the left, if any. In this case, for 128×128 rootSBs, there can be a “delay” region of 256×256 within which the referenceblock cannot be located, which further hurts compression efficiency.

FIG. 9b shows an example (902) of another constraint that can help ahardware-based encoder or hardware-based decoder avoid inefficientpatterns of memory access during intra BC prediction operations, with amuch smaller penalty on compression efficiency. The current picturebeing encoded/decoded has multiple root SBs, including the current rootSB (910) being encoded/decoded. The current block (920) is part of thecurrent root SB (910). A BV value (941) indicates the location of acandidate reference block in an outer search area (931), which includesreconstructed content of previous root SBs of the current picture, butnot the current root SB (910). The BV value (941) can indicate thelocation of a candidate reference block anywhere in the outer searcharea (931). Another BV value (942) indicates the location of a candidatereference block in an inner search area (932), which includesreconstructed parts of the current root SB (930). A candidate referenceblock can be totally outside the inner search area (932) or totallyinside the inner search area (932), but a candidate reference blockcannot be partly inside and partly outside the inner search area (932).Thus, a candidate reference block cannot cross the boundary line betweenthe inner search area (932) and outer search area (931).

FIG. 10 further illustrates this constraint. In FIG. 10, a boundary line(1011) separates an outer search area (1010) and an inner search area(1012). Different BV values for a current block (1020) beingencoded/decoded with intra BC prediction are shown. One BV value (1041)references a valid candidate reference block located entirely within theouter search area (1010). Another BV value (1042) references a validcandidate reference block located entirely within the inner search area(1012). The first and second BV values (1041, 1042) satisfy theconstraint. A third BV value (1043), however, references an invalidcandidate reference block that overlaps the inner search area (1012) andouter search area (1010). The invalid candidate reference block crossesthe boundary line (1011). Thus, the third BV value (1043) fails theconstraint.

In typical hardware-based implementations, reconstructed sample valuesinside the inner search area are buffered in on-chip memory, andreconstructed sample values inside the outer search area are buffered inoff-chip memory. The constraint illustrated in FIGS. 9b and 10 isbeneficial in terms of memory access efficiency for intra BC predictionoperations. Reconstructed sample values of a reference block are locatedonly in on-chip memory or only in off-chip memory, but do not straddleboth on-chip memory and off-chip memory. At the same time, referenceblocks within the inner search area can be close to the current blockbeing encoded/decoded, which can improve compression efficiency comparedto the constraint illustrated in FIG. 9a . Although reference blocksthat overlap the inner search area and outer search area are notallowed, the resulting penalty on compression efficiency is usuallysmall.

In the example (902) of FIG. 9b , the inner search area (932) is thecurrent root SB (910). Alternatively, the inner search area (932) canhave another size (e.g., the current root SB (910) and previous root SBto the left, if any). For other implementations, the inner search area(932) can be defined differently. For example, in extensions of theH.265 standard, the inner search area is the current CTU. Or, the innersearch area is the current CTU and previous CTU to the left, if any.Alternatively, the inner search area can have a size and/or shape thatdoes not align with root SB/CTU boundaries.

More generally, the boundary between the inner search area and outersearch area can be pre-defined (e.g., one root unit, two root units).Or, the boundary can be signaled with a syntax element in a bitstream,allowing the inner search area to have a flexible size. For example,different complexity levels for decoding, associated with different setsof resources expected to be available for decoding, could have differentsizes of inner search area.

In FIGS. 9b and 10, a boundary line cleanly separates the inner searcharea from the outer search area. In this case, there is no gap regionbetween the inner search area and the outer search area. Alternatively,a boundary region with a width of one or more sample values can separatethe inner search area and the outer search area.

In general, the inner search area is set for encoding/decoding of allunits within a root unit (e.g., root SB, CTU). Even if the root unit ispartitioned into smaller units for encoding/decoding, the inner searcharea does not shift in size or location for encoding/decoding of therespective smaller units within the root unit. This means that differentparts of the inner search area may be available or not available for usein intra BC prediction, depending on the location of the current unit.As shown in FIG. 9b , for the current block (920), the inner search area(932) includes some not-yet-coded parts (912) of the current root SB(910), which are off limits for intra BC prediction of the current block(920). The encoder can apply availability checking rules, as describedbelow, within the inner search area to determine whether a candidatereference block in the inner search area includes reconstructed samplevalues available for use in intra BC prediction.

When the inner search area is a root unit (e.g., root SB, CTU), therecan be overlap between the reference block and root unit. In some cases,reconstructed sample values in the root unit may be used for predictionof sample values of a block within the root unit. Even so, there is nooverlap between the reference block and current block beingencoded/decoded.

The constraints described in this section can be enforced in conjunctionwith other constraints as described in the next section. For example,for a candidate reference block in an outer search area, availabilityrules can be checked for rows of root units, range constraints for BVvalues can be enforced, and/or constraints on locations of candidatereference blocks for parallel processing can be enforced. For acandidate reference block in an inner search area, availability rulescan be checked within the inner search area. Range constraints on BVvalues and constraints on locations of candidate reference blocks forparallel processing may be checked, or may be inapplicable given thepossible displacements within the inner search area.

E. Examples of Other Constraints on BV Values.

An encoder can enforce various other constraints on the location of areference block when determining the BV value associated with a currentunit of a picture. This section details examples of other constraintsthat an encoder can enforce for intra BC prediction. Some constraintscan ensure that a reference block indicated by a BV value includesreconstructed sample values that will be available when the currentblock is encoded or decoded. Still other constraints help limit bit rateassociated with signaling BV values or enable parallel encoding/decodingoperations for intra BC prediction. Constraints enforced by an encoderon allowable BV values can alternatively be expressed in terms ofbitstream conformance requirements for BV values in a codec format.

Suppose a current block of a current unit starts from position (x_(curr)_(_) _(block), y_(curr) _(_) _(block)), relative to the top-leftposition of the current picture. The width and height of the currentblock (and reference block) are w_(block) and h_(block), respectively.The current block (and current unit) can be part of a root unit (e.g.,128×128 SB, 64×64 CTU). The root unit size is S. The block vector usedin intra BC prediction for the current (luma) block of the current unitis (BV_(x), BV_(y)).

1. Example Constraints on Range of BV Values.

To limit the bit rate cost associated with signaling of BV values, therange of BV values can be constrained. The range of BV values can be thesame horizontally and vertically, or the range of BV values can bedifferent horizontally and vertically. That is, BV_(min) _(_)_(x)<=BV_(x)<=BV_(max) _(_) _(x), and BV_(min) _(_)_(y)<=BV_(y)<=BV_(max) _(_) _(y), where BV_(min) _(_) _(x) and BV_(max)_(_) _(x) indicate the minimum value and maximum value, respectively,allowed for the horizontal component of a BV value, and BV_(min) _(_)_(y) and BV_(max) _(_) _(y) indicate the minimum value and maximumvalue, respectively, allowed for the vertical component of a BV value.For example, the range is [−1023, 1023] for the horizontal component ofa BV value, and the range is [−1023, 124] for the vertical component ofa BV value. Alternatively, the horizontal component and/or verticalcomponent of a BV value can have other ranges.

2. Example Constraints for Tiles or Slices.

A reference block can be constrained to be within the same tile as thecurrent block. In this case, intra BC prediction does not use samplevalues in other tiles. Or, when slice coding is used, a reference blockcan be constrained to be within the same slice as the current block. Inthis case, intra BC prediction does not use sample values in otherslices.

For example, the encoder verifies that the position (x_(curr) _(_)_(block), y_(curr) _(_) _(block)) and the position (x_(curr) _(_)_(block)+BV_(x), y_(curr) _(_) _(block)+BV_(y)) are in the same slice,if applicable, and in the same tile, if applicable. That is, the encoderverifies that the top-left position of the current block and thetop-left position of the reference block are in the same slice and inthe same tile. If the two positions are in different slices or differenttiles, the constraint is not satisfied. The encoder also verifies thatthe position (x_(curr) _(_) _(block), y_(curr) _(_) _(block)) and theposition (x_(curr) _(_) _(block)+BV_(x)+w_(block)−1, y_(curr) _(_)_(block)+BV_(y)+h_(block)−1) are in the same slice, if applicable, andsame tile, if applicable. That is, the encoder verifies that thetop-left position of the current block and the bottom-right position ofthe reference block are in the same slice and in the same tile. If thetwo positions are in different slices or different tiles, the constraintis not satisfied. All positions of the current block are in a singleslice and single tile. If these constraints are satisfied, all positionsof the reference block are also in that slice and tile.

If multiple slices are not used, the two positions that are checked arenecessarily in the same slice, and the constraints for slices need notbe checked, although the reference block is still constrained to bewithin the current picture. Similarly, if multiple tiles are not used,the two positions that are checked are necessarily in the same tile, andthe constraints for tiles need not be checked, although the referenceblock is still constrained to be within the current picture.

3. Example Constraints Based on Availability of Reconstructed SampleValues.

For intra BC prediction, the encoder and decoder use reconstructedsample values in the current picture. Unreconstructed sample valuesmight be present as parts of the current picture that have not beenreconstructed yet. To avoid using unreconstructed sample values forintra BC prediction, the encoder can set constraints on allowable BVvalues such that only actual, previously reconstructed sample values inthe current picture are used for intra BC prediction.

For example, the encoder can check whether (y_(curr) _(_)_(block)+BV_(y)+h_(block)−1)/S<y_(curr) _(_) _(block)/S. That is, theencoder calculates the root unit row that includes the bottom edge ofthe reference block: (y_(curr) _(_) _(block)+BV_(y)+h_(block)−1)/S. Theencoder also calculates the root unit row that includes the top edge ofthe current block: y_(curr) _(_) _(block)/S. The encoder then checkswhether the root unit row that includes the bottom edge of the referenceblock is above the root unit row that includes the top edge of thecurrent block. If so, since root unit rows are reconstructed fromtop-to-bottom, the reference block is within a previous root unit rowthat has already been reconstructed, and the reference block necessarilyincludes previously reconstructed sample values, at least when WPP isnot enabled.

On the other hand, when (y_(curr) _(_)_(block)+BV_(y)+h_(block)−1)/S==y_(curr) _(_) _(block)/S, the root unitrow that includes the bottom edge of the reference block equals the rootunit row that includes the top edge of the current block. This means thecurrent block is in the same root unit row as at least a bottom part ofthe reference block. In this case, the encoder checks whether (x_(curr)_(_) _(block)+BV_(x)+w_(block)−1)/S<x_(curr) _(_) _(block)/S. That is,the encoder calculates (a) the root unit column that includes the rightedge of the reference block ((x_(curr) _(_)_(block)+BV_(x)+w_(block)−1)/S), and (b) the root unit column thatincludes the left edge of the current block (x_(curr) _(_) _(block)/S).The encoder then checks whether the root unit column that includes theright edge of the reference block is left of the root unit column thatincludes the left edge of the current block. If so, since root units arereconstructed from left-to-right in a root unit row, the reference blockis within a previous root unit column that has already beenreconstructed, and the reference block necessarily includes previouslyreconstructed sample values.

On the other hand, when (y_(curr) _(_)_(block)+BV_(y)+h_(block)−1)/S==y_(curr) _(_) _(block)/S and (x_(curr)_(_) _(block)+BV_(x)+w_(block)−1)/S==x_(curr) _(_) _(block)/S, the rootunit row that includes the bottom edge of the reference block equals theroot unit row that includes the top edge of the current block, and theroot unit column that includes the right edge of the reference blockequals the root unit column that includes the left edge of the currentblock. This means the current block is in the same root unit row andsame root unit column as at least a bottom-right part of the referenceblock. In this case, the encoder checks whether the z-scan order of theposition (x_(curr) _(_) _(block)+BV_(x)+w_(block)−1, y_(curr) _(_)_(block)+BV_(y)+h_(block)−1) is smaller than the z-scan order of theposition (x_(curr) _(_) _(block), y_(curr) _(_) _(block)). That is, theencoder checks whether the bottom-right position of the reference blockis earlier in z-scan order than the top-left position of the currentblock. If so, the block that contains the bottom-right position of thereference block has been previously reconstructed, and hence so has therest of the reference block. This condition applies if prediction fromwithin a current root unit is allowed, e.g., when the current root unitdefines an inner search area for BV estimation. The encoder can alsocheck that the offset value satisfies at least one of the conditionsBV_(x)+w_(block)≤0 and BV_(y)+h_(block)≤0, ensuring that the referenceblock does not overlap the current block.

In general, the z-scan order for root units within a picture, and forpartitioned units within a root unit, follows a sequentially specifiedordering of units. FIG. 11 shows example z-scan order (1100) for acurrent unit (1130) and units that might include the bottom-rightposition of a reference block within a root SB (1110). Within a picture,the z-scan order proceeds root unit-to-root unit along a raster scanpattern (left-to-right in a root unit row, repeating in successive rootunit rows from top-to-bottom). If a root unit is partitioned intosmaller units, the z-scan order follows a raster scan pattern for thesmaller units within the root unit, left-to-right for the top row thenleft-to-right for the bottom row. If a smaller unit is furtherpartitioned, the z-scan order similarly follows a raster scan patternwithin that partitioned smaller unit. In this way, z-scan orderrecursively defines an order of traversal within a root unit, which maybe recursively partitioned.

4. Example Constraints for Wavefront Parallel Processing.

In some example implementations, when wavefront parallel processing(“WPP”) is not enabled, candidate reference blocks can be anywherewithin the reconstructed content of the current picture, subject to theother constraints described above. Root units of a picture are generallycoded in a raster scan pattern from left-to-right, then fromtop-to-bottom, with additional traversal rules applying for root unitsthat are partitioned into smaller units.

When WPP is enabled, additional constraints on locations of candidatereference blocks apply. The constraints allow a decoder to decodedifferent root unit rows in parallel, with decoding of root unit rowsstarting one after another after a delay period such as two root unitsper root unit row. Intra BC prediction is allowed only from a referenceblock within the same root unit row as the current root unit beingencoded/decoded, or from a reference block within a previous root unitrow so long as the reference block is not “too far to the right” of thecurrent root unit being encoded/decoded. What qualifies as “too far tothe right” depends on how far the reference block is above the currentroot unit.

The encoder can check whether (y_(curr) _(_)_(block)+BV_(y)+h_(block)−1)/S<=y_(curr) _(_) _(block)/S. That is, theencoder calculates the root unit row that includes the bottom edge ofthe reference block: (y_(curr) _(_) _(block)+BV_(y)+h_(block)−1)/S. Theencoder also calculates the root unit row that includes the top edge ofthe current block: y_(curr) _(_) _(block)/S. The encoder then checkswhether the root unit row that includes the bottom edge of the referenceblock is less than or equal to the root unit row that includes the topedge of the current block. If so, the reference block and the currentblock are within the same root unit row, or the reference block is in aprevious unit row.

If the reference block is in a previous unit row, the encoder can checkwhether (x_(curr) _(_) _(block)+BV_(x)+w_(block)−1)/S<(x_(curr) _(_)_(block)/S)+thr_(vert). That is, the encoder calculates the root unitcolumn that includes the right edge of the reference block: (x_(curr)_(_) _(block)+BV_(x)+w_(block)−1)/S. The encoder also calculates theroot unit column that includes the left edge of the current block:(x_(curr) _(_) _(block)/S). Finally, the encoder calculates a thresholdthr_(vert) that depends on a vertical displacement value for the BVvalue associated with the current unit. For example, the thresholdthr_(vert)=2×((y_(curr) _(_) _(block)/S)−(y_(curr) _(_)_(block)+BV_(y)+h_(block)−1)/S)). In this case, the threshold increasesas vertical distance away from the current root unit increases. Ineffect, a horizontal displacement of two root units to the right isallowed for each root unit row above the current root unit. If the startof decoding of different root unit rows is staggered by a differentamount (e.g., 1 root unit, 3 root units), the threshold thr_(vert) canbe adjusted accordingly (e.g., the scaling factor 2 becomes 1 or 3,respectively). This allows the reference block to be within a previousroot unit row so long as the reference block is within a leading edge ofreconstructed content for WPP, including some root units above and tothe right of the current root unit.

FIG. 12 shows an example (1200) of search areas for intra BC predictionin parallel encoding or decoding. The search areas are consistent withconstraints on locations of candidate reference blocks with WPP enabled,for decoding staggered by two root units per row, and consistent with aconstraint that allows candidate reference blocks in an outer searcharea or inner search area but not overlapping both. For the first rootunit row, the outer search area (1211) includes previously reconstructedroot units in the first root unit row, and the inner search area (1212)includes the current root unit being encoded/decoded in the first rootunit row. For the second root unit row, the outer search area (1221)includes previously reconstructed root units in the second root unit rowas well as the outer search area (1211) of the first root unit row. Theinner search area (1222) includes the current root unit beingencoded/decoded in the second root unit row. Similarly, for the thirdroot unit row, the outer search area (1231) includes previouslyreconstructed root units in the third root unit row as well as the outersearch areas (1211, 1221) of the previous root unit rows. The innersearch area (1232) includes the current root unit being encoded/decodedin the third root unit row. Outer search areas (1241, 1251) and innersearch areas (1242, 1252) are similarly defined for the fourth and fifthroot unit rows.

Alternatively, for root unit rows whose decoding is staggered by oneroot unit, the encoder can verify that (x_(curr) _(_)_(block)+BV_(x)+w_(block)−1)/S−x_(curr) _(_) _(block)/S<y_(curr) _(_)_(block)/S−(y_(curr) _(_) _(block)+BV_(y)+h_(block)−1)/S. That is, theencoder calculates the difference between the root unit column thatincludes the right edge of the reference block and the root unit columnthat includes the left edge of the current block: (x_(curr) _(_)_(block)+BV_(x)+w_(block)−1)/S−x_(curr) _(_) _(block)/S. The encoderalso calculates the difference between the root unit row that includesthe top edge of the current block and the root unit row that includesthe bottom edge of the reference block:y_(cb)/S−(y_(cb)+BV_(y)+h_(block)−1)/S. The encoder verifies that thefirst difference (between root unit columns) is less than the seconddifference (between root unit rows). More generally, a horizontaldisplacement value from a reference block to the current block isconstrained to be less than or equal to a vertical displacement valuefrom the current block to the reference block.

F. Example Techniques for Encoding with Constraints on Locations ofReference Blocks for Intra BC Prediction Mode.

FIG. 13 shows a generalized technique (1300) for encoding withconstraints on locations of reference blocks for intra BC predictionmode. A video encoder such as one described with reference to FIG. 3 orFIGS. 4a-4b , or other video encoder or image encoder, can perform thetechnique (1300).

At a high level, the encoder encodes (1310) a current picture to produceencoded data and outputs (1320) the encoded data as part of a bitstream.During the encoding (1310), for a current unit (e.g., superblock,macroblock, prediction unit) of the current picture, the encoderdetermines a BV value associated with the current unit. The BV valueindicates a displacement from a given block of the current unit to areference block in the current picture. The encoder performs intra BCprediction for the given block of the current unit using the BV valueassociated with the current unit.

When determining the BV value associated with the current unit, theencoder enforces a constraint that allows the reference block to beentirely within an inner search area of the current picture and allowsthe reference block to be entirely within an outer search area of thecurrent picture but prohibits the reference block from overlapping boththe inner search area and the outer search area. For example, theconstraint is one of the constraints described in section V.D. Aboundary line or boundary region can separate the inner search area fromthe outer search area. To enforce this constraint, the encoder can checkthat the reference block does not cross the boundary line or boundaryregion. Or, to enforce this constraint, the encoder can check that thereference block is completely contained within the inner search area(e.g., at or inside a perimeter of the inner search area) or the outersearch area (e.g., at or inside a perimeter of the outer search area butoutside the inner search area).

Enforcing this constraint can make intra BC prediction more efficientfor hardware implementations in which memory includes on-chip memory andoff-chip memory. In some example implementations, a first buffer (whichis part of the on-chip memory) stores sample values of the inner searcharea, and a second buffer (which is part of the off-chip memory) storessample values of the outer search area. After reconstruction of all ofthe sample values of the inner search area, the encoder can transfer thesample values of the inner search area from the first buffer to thesecond buffer.

The encoder can also enforce one or more other constraints whendetermining the BV value associated with the current unit. For example,the encoder can enforce a constraint that the BV value associated withthe current unit be within a range of permissible BV values. SectionsV.B and V.E describe examples of range constraints for BV values. Or, asanother example, with a test for z-scan order, the encoder can enforce aconstraint that the reference block include sample values that have beenreconstructed. Section V.E describes examples of constraints thatconsider availability with a raster scan pattern or z-scan pattern. Or,as another example, the encoder can enforce a constraint that thereference block not violate a rule on dependencies between differentsections of the current picture for parallel encoding or decoding.Section V.E describes examples of constraints for wavefront parallelprocessing. Or, as another example, the encoder can enforce a constraintthat the given block and the reference block be in the same tile, ifapplicable, and in the same slice, if applicable. Section V.E describesexamples of constraints for tile boundaries and slice boundaries. Or, asanother example, the encoder can enforce some other constraint.

Typically, the given block of the current unit is a primary componentblock. The primary component block can include luma sample values. Whenthe current unit also includes secondary component blocks with samplevalues (e.g., chroma sample values) that have been sub-sampled relativeto the sample values of the primary component block, the encoder canderive a BV value for the secondary component blocks of the current unitbased at least in part on the BV value associated with the current unit,then perform intra BC prediction for the secondary component blocksusing the derived BV value. For example, if the current picture is in aYUV 4:2:0 format, the BV value can be sub-sampled by a factor of twohorizontally and vertically.

To determine the BV value associated with the current unit, the encodercan use any of the BV estimation strategies described in section V.C.For example, the encoder uses hash-based block matching and/or blockmatching according to a search pattern to determine the BV valueassociated with the current unit. Alternatively, the encoder can useanother BV estimation strategy.

The BV value associated with the current unit has an integer-samplevalue resolution. That is, the horizontal component of the BV value isan integer number of sample values, and the vertical component of the BVvalue is an integer number of sample values. Alternatively, the BV valueassociated with the current unit can have a fractional-sample valueresolution. In this case, an interpolation filter is applied toreconstructed sample values at integer sample value locations todetermine predicted sample values. The interpolation filter has a filtersupport region, which includes sample values used in interpolation ofsample values of the reference block. The filter support region includessome sample values just outside the reference block. When determiningthe BV value associated with the current unit, the encoder can enforce aconstraint that allows the filter support region around the referenceblock to be entirely within the inner search area and allows the filtersupport region to be entirely within the outer search area but prohibitsthe filter support region from overlapping both the inner search areaand the outer search area.

The current picture includes multiple root units. Each of the root unitshas a maximum unit size and can be partitioned into smaller units. Forexample, for extensions of the AV1 format, each of the root units is a128×128 SB. Or, for extensions of the H.265 format, each of the rootunits is a 64×64 CTU. When the encoder determines the BV valueassociated with the current unit, the current unit can be one of themultiple root units. Or, a given root unit can be partitioned intosmaller units, and the current unit is one of the smaller units. Thus,for example, the current unit can have a size of 256×256, 256×128,128×256, 128×128, 128×64, 64×128, 64×64, 64×32, 32×64, 32×32, 32×16,16×32, 16×16, 16×8, 8×16, 8×8, 8×4, 4×8, or 4×4 sample values.

In some example implementations, the inner search area is a root unit(e.g., 128×128 SB). That is, the inner search area has a size equal tosize of the root unit. When a root unit is partitioned into smallerunits, the inner search area for the smaller units is still the rootunit. That is, the inner search area is unchanged for any and all of thesmaller units of the root unit.

Alternatively, the inner search can be multiple root units (e.g.,current 128×128 SB and previous 128×128 SB to the left). Thus, forexample, the inner search area can have a height equal to the height ofa root unit and a width double the width of a unit. The inner searcharea is unchanged for smaller units of a root unit.

The size of the inner search area can be pre-defined. For example, thesize of the inner search area can be defined as a single root unit ortwo root units. Or, the size of the inner search area can be indicatedby a syntax element in the bitstream. The syntax element in thebitstream can indicate one of multiple possible sizes for the innersearch area, or the syntax element can expressly specify the size of theinner search area. For example, the syntax element is signaled as partof level information that specifies resource requirements for a decoder.

FIG. 14 shows an example technique (1400) for encoding units of SBs of acurrent picture with intra BC prediction. A video encoder such as onedescribed with reference to FIG. 3 or FIGS. 4a-4b , or other videoencoder or image encoder, can perform the technique (1400).

The encoder checks (1410) whether a new root SB of the current picturehas been reached. If so, the encoder sets (1420) a new inner search areafor units of the new root SB. For example, the inner search area is the128×128 region for the root SB.

The encoder gets (1430) the current unit of the root SB, which is thenext unit to be encoded in the root SB. The encoder determines (1440) aBV value associated with the current unit, subject to one or moreconstraints on the BV value. The constraint(s) include a constraint thatallows the reference block to be entirely within an inner search area ofthe current picture and allows the reference block to be entirely withinan outer search area of the current picture but prohibits the referenceblock from overlapping both the inner search area and the outer searcharea. For example, the encoder determines (1440) the BV value associatedwith the current unit as shown in FIGS. 15a and 15b , which show exampletechniques (1501, 1502) for determining the BV value. Alternatively, theencoder determines (1440) the BV value associated with the current unitin some other way.

With reference to FIG. 15a , the encoder finds (1510) the next “best” BVvalue associated with the current unit using BV estimation. Section V.Cdescribes examples of approaches for BV estimation. The criteria forwhat constitutes a “best” BV value can account for sample valuedifferences (error) between the corresponding reference block andcurrent block as well as bit rate associated with signaling the BVvalue, before enforcing constraints on the BV value. The encoder thenchecks (1520) whether the reference block for the BV value crosses aboundary between the inner search area and outer search area of thecurrent picture. If the reference block crosses the boundary, thecurrent “best” BV value is discarded, and the encoder finds (1510) thenext “best” BV value associated with the current unit. Otherwise (the BVvalue satisfies the boundary constraint), the encoder checks (1530) oneor more other constraints on the BV value. For example, the encoderchecks one or more of the other constraints described in section V.E. Ifthe BV value satisfies the other constraint(s), the BV value is used forthe current unit. Otherwise (the BV value fails at least one of theconstraint(s)), the current “best” BV value is discarded, and theencoder finds (1510) the next “best” BV value associated with thecurrent unit.

With reference to FIG. 15b , the encoder finds (1550) the next “best” BVvalue associated with the current unit in the outer search area using BVestimation, and also finds (1560) the next “best” BV value associatedwith the current unit in the inner search area using BV estimation.Neither of the “best” BV values can possibly cross a boundary betweenthe inner search area and outer search area. Section V.C describesexamples of approaches for BV estimation. The encoder selects (1570)between the “best” BV value associated with the current unit in theouter search area and the “best” BV value associated with the currentunit in the inner search area. The criteria for what constitutes a“better” or “best” BV value can account for sample value differences(error) between the corresponding reference block and current block aswell as bit rate associated with signaling the BV value, beforeenforcing constraints on the BV value. The encoder checks (1580) one ormore constraints on the BV value. For example, the encoder checks one ormore of the other constraints described in section V.E. If the BV valuesatisfies the constraint(s), the BV value is used for the current unit.Otherwise (the BV value fails at least one of the constraint(s)), theselected “best” BV value is discarded, and the encoder finds (1550,1560) the next “best” BV values associated with the current unit in theouter search area and inner search area, respectively.

Returning to FIG. 14, the encoder performs (1450) intra BC predictionfor the current unit using the BV value and optionally encodes (1460)residual values for the current unit. The encoder also reconstructssample values for the current unit in on-chip memory.

The encoder checks (1470) if the last unit of the root SB has beenencoded. If so, the encoder transfers (1480) reconstructed sample valuesfor the root SB from on-chip memory to off-chip memory.

The encoder checks (1490) whether to continue for the next unit of thecurrent picture. If so, the encoder continues by checking (1410) whethera new root SB has been reached. The encoder can repeat the technique(1400) on a picture-by-picture basis.

G. Example Techniques for Decoding with Constraints on Locations ofReference Blocks for Intra BC Prediction Mode.

FIG. 16 shows a generalized technique (1600) for decoding withconstraints on locations of reference blocks for intra BC predictionmode. A decoder such as one described with reference to FIG. 5 or FIG.6, or other video decoder or image decoder, can perform the technique(1600).

At a high level, the decoder receives (1610) encoded data as part of abitstream. The encoded data satisfies various bitstream conformancerequirements. The decoder decodes (1620) a current picture using theencoded data. During the decoding (1620), for a current unit (e.g.,superblock, macroblock, prediction unit) of the current picture, thedecoder reconstructs a BV value associated with the current unit. The BVvalue indicates a displacement from a given block of the current unit toa reference block in the current picture. The decoder performs intra BCprediction for the given block of the current unit using the BV valueassociated with the current unit.

According to one of the bitstream conformance requirements, the BV valueis constrained so that the reference block is allowed to be entirelywithin an inner search area of the current picture and the referenceblock is allowed to be entirely within an outer search area of thecurrent picture, but the reference block is prohibited from overlappingboth the inner search area and the outer search area. Enforcement ofthis constraint can make intra BC prediction more efficient for hardwareimplementations in which memory includes on-chip memory and off-chipmemory. In some example implementations, a first buffer (which is partof the on-chip memory) stores sample values of the inner search area,and a second buffer (which is part of the off-chip memory) stores samplevalues of the outer search area. After reconstruction of all of thesample values of the inner search area, the decoder can transfer thesample values of the inner search area from the first buffer to thesecond buffer.

According to one or more other bitstream conformance requirements, theBV value associated with the current unit can be constrained in variousother ways. For example, the BV value can be constrained so that it iswithin a range of permissible BV values. Sections V.B and V.E describeexamples of range constraints for BV values. Or, as another example, theBV value can be constrained so that, according to a test for z-scanorder, the reference block includes sample values that have beenreconstructed. Section V.E describes examples of constraints thatconsider availability with a raster scan pattern or z-scan pattern. Or,as another example, the BV value can be constrained so that thereference block does not violate a rule on dependencies betweendifferent sections of the current picture for parallel decoding. SectionV.E describes examples of constraints for wavefront parallel processing.Or, as another example, the BV value can be constrained so that thegiven block and the reference block are in the same tile, if applicable,and in the same slice, if applicable. Section V.E describes examples ofconstraints for tile boundaries and slice boundaries. Or, as anotherexample, the BV value can be constrained in some other way.

Typically, the given block of the current unit is a primary componentblock. The primary component block can include luma sample values. Whenthe current unit also includes secondary component blocks with samplevalues (e.g., chroma sample values) that have been sub-sampled relativeto the sample values of the primary component block, the decoder canderive a BV value for the secondary component blocks of the current unitbased at least in part on the BV value associated with the current unit,then perform intra BC prediction for the secondary component blocksusing the derived BV value. For example, if the current picture is in aYUV 4:2:0 format, the BV value can be sub-sampled by a factor of twohorizontally and vertically.

The BV value associated with the current unit has an integer-samplevalue resolution. That is, the horizontal component of the BV value isan integer number of sample values, and the vertical component of the BVvalue is an integer number of sample values. Alternatively, the BV valueassociated with the current unit can have a fractional-sample valueresolution. In this case, the BV value associated with the current unitcan be constrained so that a filter support region around a referenceblock is entirely within the inner search area or entirely within theouter search area, but the filter support region is prohibited fromoverlapping both the inner search area and the outer search area.

The current picture includes multiple root units. Each of the root unitshas a maximum unit size and can be partitioned into smaller units. Forexample, for extensions of the AV1 format, each of the root units is a128×128 SB. Or, for extensions of the H.265 format, each of the rootunits is a 64×64 CTU. When the decoder reconstructs the BV valueassociated with the current unit, the current unit can be one of themultiple root units. Or, a given root unit can be partitioned intosmaller units, and the current unit is one of the smaller units. Thus,for example, the current unit can have a size of 256×256, 256×128,128×256, 128×128, 128×64, 64×128, 64×64, 64×32, 32×64, 32×32, 32×16,16×32, 16×16, 16×8, 8×16, 8×8, 8×4, 4×8, or 4×4 sample values.

In some example implementations, the inner search area is a root unit(e.g., 128×128 SB). That is, the inner search area has a size equal tosize of the root unit. When a root unit is partitioned into smallerunits, the inner search area for the smaller units is still the rootunit. That is, the inner search area is unchanged for any and all of thesmaller units of the root unit.

Alternatively, the inner search can be multiple root units (e.g.,current 128×128 SB and previous 128×128 SB to the left). Thus, forexample, the inner search area can have a height equal to the height ofa root unit and a width double the width of a root unit. The innersearch area is unchanged for smaller units of a root unit.

The size of the inner search area can be pre-defined. For example, thesize of the inner search area can be defined as a single root unit ortwo root units. Or, the size of the inner search area can be indicatedby a syntax element in the bitstream. The syntax element in thebitstream can indicate one of multiple possible sizes for the innersearch area, or the syntax element can expressly specify the size of theinner search area. For example, the syntax element is signaled as partof level information that specifies resource requirements for a decoder.

H. Alternatives and Variations.

In many of the examples described herein, an encoder enforcesconstraints on allowable BV values. The constraints can alternatively beconsidered constraints on values of syntax elements in a bitstream orconstraints on BV values when reconstructed during decoding, where theencoder still selects the BV values. That is, the constraints can beexpressed as bitstream conformance requirements for BV values in encodeddata in a bitstream.

In many of the examples described herein, intra BC prediction and motioncompensation are implemented in separate components or processes, and BVestimation and motion estimation are implemented in separate componentsor processes. Alternatively, intra BC prediction can be implemented as aspecial case of motion compensation, and BV estimation can beimplemented as a special case of motion estimation, for which thecurrent picture is used as a reference picture. In such implementations,a BV value can be signaled as an MV value but used for intra BCprediction (within the current picture) rather than inter-pictureprediction. As the term is used herein, “intra BC prediction” indicatesprediction within a current picture, whether that prediction is providedusing an intra-picture prediction module, a motion compensation module,or some other module. Similarly, a BV value can be represented using anMV value or using a distinct type of parameter or syntax element, and BVestimation can be provided using an intra-picture estimation module,motion estimation module or some other module.

For a BV value associated with a unit, intra BC prediction operationscan be applied at the level of a given block that has the same spatialresolution as the unit. In this case, a reference block is constrainedto not overlap the given block. Alternatively, intra BC predictionoperations can be applied for smaller blocks within the unit. Forexample, for a first smaller block within the unit, the reference blockincludes positions outside the unit. For a second smaller block withinthe unit, however, the reference block used in intra BC predictionoperations can include positions in the previously reconstructed firstsmaller block. In this way, since reconstruction rules apply the BVvalue for intra BC prediction of smaller blocks, the BV value canreference positions in the same unit being encoded/decoded, and thereference block may overlap the unit. Allowing intra BC predictionoperations to be applied for smaller blocks within a unit facilitatesuse of BV values with relatively small magnitudes.

In many of the examples described herein, a reference block for intra BCprediction is entirely within a current picture (or entirely within atile, if tile constraints are used; or entirely within a slice, if sliceconstraints are used). Alternatively, a reference block for intra BCprediction can be at least partially outside the current picture (ortile, or slice). In this case, sample values can be padded away from theboundary of the current picture (or tile, or slice) to providereconstructed sample values for the reference block. Or, default samplevalues can be used to provide reconstructed sample values for thereference block that are outside the boundary of the current picture (ortile, or slice).

In many of the examples described herein, the shape of a current blockand reference block are rectilinear (e.g., square, rectangle)—thecurrent block is a current rectangle, the reference block is a referencerectangle, and the intra BC prediction is intra rectangle copyprediction for the current rectangle and the reference rectangle.Alternatively, the shape of the current block and reference block can bea line—the current block is a current line, the reference block is areference line, and the intra BC prediction is intra line copyprediction for the current line and the reference line. Determining theBV value associated with the current unit includes setting an offsetvalue from the current line to the reference line, which includespreviously reconstructed sample values that are scanned in a linescanning direction. Or, as another alternative, the shape of the currentblock and reference block can be a string in a rectangle—the currentblock is a current string, the reference block is a reference string,and the intra BC prediction is intra string copy prediction for thecurrent string and the reference string. Determining the BV valueassociated with the current unit includes: (a) identifying a rectanglethat includes the current string by determining a rectangle thatincludes a start position of the current string, an end position of thecurrent string, and any positions between the start position and the endposition of the current string in a string scanning order; (b) settingan offset value, applied to the rectangle that includes the currentstring, to a rectangle that includes the reference string, whichincludes previously reconstructed sample values that are scanned in thestring scanning order; and (c) setting a length value that indicateslength for each of the current string and the reference string.

In view of the many possible embodiments to which the principles of thedisclosed invention may be applied, it should be recognized that theillustrated embodiments are only preferred examples of the invention andshould not be taken as limiting the scope of the invention. Rather, thescope of the invention is defined by the following claims. We thereforeclaim as our invention all that comes within the scope and spirit ofthese claims.

We claim:
 1. A computer system comprising one or more processing coresand memory, wherein the computer system is configured to implement avideo encoder or image encoder that performs operations comprising:encoding a current picture to produce encoded data, including, for acurrent unit of the current picture: determining a block vector (“BY”)value associated with the current unit, the BV value indicating adisplacement from a given block of the current unit to a reference blockin the current picture, wherein the determining the BV value includesenforcing a constraint that allows the reference block to be entirelywithin an inner search area of the current picture and allows thereference block to be entirely within an outer search area of thecurrent picture but prohibits the reference block from overlapping boththe inner search area and the outer search area; and performing intrablock copy (“BC”) prediction for the given block of the current unitusing the BV value associated with the current unit; and outputting theencoded data as part of a bitstream.
 2. The computer system of claim 1,wherein the current unit is a superblock, a macroblock, or a predictionunit, wherein the given block of the current unit is a primary componentblock, wherein the current unit also includes secondary componentblocks, and wherein the encoding further includes, for the current unit:deriving a BV value for the secondary component blocks of the currentunit based at least in part on the BV value associated with the currentunit; and performing intra BC prediction for the secondary componentblocks using the derived BV value.
 3. The computer system of claim 2,wherein the primary component block includes luma sample values and thesecondary component blocks include chroma sample values that have beensub-sampled relative to the luma sample values.
 4. The computer systemof claim 1, wherein the determining the BV value associated with thecurrent unit uses hash-based block matching and/or block matchingaccording to a search pattern.
 5. The computer system of claim 1,wherein the determining the BV value associated with the current unitfurther includes: enforcing a constraint that the BV value associatedwith the current unit be within a range of permissible BV values; with atest for z-scan order, enforcing a constraint that the reference blockinclude sample values that have been reconstructed; enforcing aconstraint that the reference block not violate a rule on dependenciesbetween different sections of the current picture for parallel encodingor decoding; and/or enforcing a constraint that the given block and thereference block be in the same tile, if applicable, and in the sameslice, if applicable.
 6. The computer system of claim 1, wherein aboundary line or boundary region separates the inner search area fromthe outer search area, and wherein the enforcing the constraintincludes: checking that the reference block does not cross the boundaryline or boundary region; or checking that the reference block iscompletely contained within the inner search area or within the outersearch area.
 7. The computer system of claim 1, wherein the currentpicture includes multiple root units, wherein each of the multiple rootunits has a maximum unit size and can be partitioned into smaller units,and wherein a given root unit of the multiple root units includes thecurrent unit, the inner search area being unchanged for all of the unitsof the given root unit.
 8. The computer system of claim 7, wherein theinner search area has a size equal to size of the given root unit. 9.The computer system of claim 1, wherein the inner search area has a sizethat is: pre-defined; or indicated by a syntax element in the bitstream.10. The computer system of claim 1, wherein a first buffer stores samplevalues of the inner search area, wherein a second buffer stores samplevalues of the outer search area, and wherein the encoding furtherincludes: after reconstruction of all of the sample values of the innersearch area, transferring the sample values of the inner search areafrom the first buffer to the second buffer.
 11. The computer system ofclaim 10, wherein the memory of the computer system includes on-chipmemory and off-chip memory, wherein the first buffer is part of theon-chip memory, and wherein the second buffer is part of the off-chipmemory.
 12. In a computer system comprising one or more processing coresand memory, a method of video decoding or image decoding, the methodcomprising: receiving encoded data as part of a bitstream, wherein theencoded data satisfies bitstream conformance requirements; and decodinga current picture using the encoded data, including, for a current unitof the current picture: reconstructing a block vector (“BY”) valueassociated with the current unit, the BV value indicating a displacementfrom a given block of the current unit to a reference block in thecurrent picture, wherein, according to one of the bitstream conformancerequirements, the BV value is constrained so that the reference block isallowed to be entirely within an inner search area of the currentpicture and the reference block is allowed to be entirely within anouter search area of the current picture but the reference block isprohibited from overlapping both the inner search area and the outersearch area; and performing intra block copy (“BC”) prediction for thegiven block of the current unit using the BV value associated with thecurrent unit.
 13. The method of claim 12, wherein the current unit is asuperblock, a macroblock, or a prediction unit, wherein the given blockof the current unit is a primary component block, wherein the currentunit also includes secondary component blocks, and wherein the decodingfurther includes, for the current unit: deriving a BV value for thesecondary component blocks of the current unit based at least in part onthe BV value associated with the current unit; and performing intra BCprediction for the secondary component blocks using the derived BVvalue.
 14. The method of claim 12, wherein, according to the bitstreamconformance requirements, the BV value associated with the current unitis further constrained so that: the BV value associated with the currentunit is within a range of permissible BV values; according to a test forz-scan order, the reference block includes sample values that have beenreconstructed; the reference block does not violate a rule ondependencies between different sections of the current picture forparallel decoding; and/or the given block and the reference block are inthe same tile, if applicable, and in the same slice, if applicable. 15.The method of claim 12, wherein the BV value associated with the currentunit has integer-sample value resolution, and wherein the current unithas a size of 256×256, 256×128, 128×256, 128×128, 128×64, 64×128, 64×64,64×32, 32×64, 32×32, 32×16, 16×32, 16×16, 16×8, 8×16, 8×8, 8×4, 4×8, or4×4 sample values.
 16. The method of claim 12, wherein the currentpicture includes multiple root units, wherein each of the multiple rootunits has a maximum unit size and can be partitioned into smaller units,and wherein a given root unit of the multiple root units includes thecurrent unit, the inner search area being unchanged for all of the unitsof the given root unit.
 17. The method of claim 12, wherein the innersearch area has a size that is: pre-defined; or indicated by a syntaxelement in the bitstream.
 18. The method of claim 12, wherein a firstbuffer stores sample values of the inner search area, wherein a secondbuffer stores sample values of the outer search area, and wherein thedecoding further includes: after reconstruction of all of the samplevalues of the inner search area, transferring the sample values of theinner search area from the first buffer to the second buffer.
 19. Themethod of claim 18, wherein the memory of the computer system includeson-chip memory and off-chip memory, wherein the first buffer is part ofthe on-chip memory, and wherein the second buffer is part of theoff-chip memory.
 20. One or more computer-readable media storingcomputer-executable instructions for causing a computer system, whenprogrammed thereby, to perform operations comprising: receiving encodeddata as part of a bitstream, wherein the encoded data satisfiesbitstream conformance requirements; and decoding a current picture usingthe encoded data, including, for a current unit of the current picture:reconstructing a block vector (“BY”) value associated with the currentunit, the BV value indicating a displacement from a given block of thecurrent unit to a reference block in the current picture, wherein,according to one of the bitstream conformance requirements, the BV valueis constrained so that the reference block is allowed to be entirelywithin an inner search area of the current picture and the referenceblock is allowed to be entirely within an outer search area of thecurrent picture but the reference block is prohibited from overlappingboth the inner search area and the outer search area; and performingintra block copy (“BC”) prediction for the given block of the currentunit using the BV value associated with the current unit.